There are already good answers above. I agree blockchain.info wallets make it easy to sweep a private key, and this probably is the simplest way.

You can do it in the Satoshi/QT client by going to Help>Debug Window and then clicking the Console tab. From there you can do what you want using createrawtransaction, signrawtransaction and sendrawtransaction. I think this will work even without downloading the blockchain, but I've never tried. For createrawtransaction you will need to know the transaction id and output of the transaction* you used to load the address on your paper wallet, and the address you want to send the money to. For signrawtransaction you will need the private key.

*or "transactions" if there were more than one.

I didn't intend for it to be like Pay What You Want, so I probably shouldn't have extended the description with different options. Let me stick with one option:

The customer pays 4 mbits and the merchant promises to refund 2 mbits if the price doubles within 6 months. Here's what should happen when the customer buys one unit. Let's assume the current price is 1 mbit = $1.

Step 1. The customer sends 4 mbits to the merchant and sends a refund address.
Step 2. The merchant cashes out 2 mbits for $2. The merchant then is ensured of having been paid at least these $2. The merchant holds onto the other 2 mbits.
Step 3. Wait 6 months. If at any time during the 6 months the price doubles, then the merchant sends the 2 mbits to the refund address.
Step 4. At the end of the 6 months, if the merchant still has the 2 mbits, then the merchant sells them at the current price. Note that this price is necessarily 1 mbit < $2, so the merchant pockets less than $4 at this step.

For the merchant, the effective price was between $2 and $6. By varying the values, the merchant can decide on a comfortable range of prices.

A merchant could charge 4 mbits (about $4 today) and promise to refund 2 mbits if the price doubles within six months. In general, a merchant could give several such options. For example.

1. Pay 3 mbits today with no possible future refunds.
2. Pay 4 mbits today with a refund of 2 mbits if the price doubles within six months.
3. Pay 5 mbits today with a refund of 2.5 mbits if the price doubles within six months and a further refund of 1.25 mbits if the price quadruples within the next year.

This amounts to building option contracts into the pricing model. The effect is to reduce the volatility risk taken on by the customer.

I don't think anyone is using my lisp code, but I wanted to post an update just in case. After the post above, I added an interface file (bip0032sbcl.lisp) and a file (make.lisp) that could be used to generate a command line executable (bip0032sbclexec). Someone could then use this executable to do BIP0032 operations without using lisp at all. There are examples in the README.md at the github link above.

I discovered a bug in the interface code that I fixed today. If anyone is using it, I strongly recommend that they get the fixed version.

Here is a bit more detail for those interested. In BIP0032, one can use extended private keys to derive two kinds of children. One uses "private derivation" and the other uses "public derivation". "private derivation" is used if the 31'st bit is set to 1. The bug was that I set the 32'nd bit to 1 instead of the 31'st. As a consequence, "public derivation" was always being used. As an example, the buggy version did this:

Ask for extended private key of private child 1 of a certain extended private key (all in hex):

Ask for extended private key of public child 1 of the extended private key:

The results are the same, while they should be different.

Retrying the operations with the corrected version, we get:


Now they're different.

Nächstes Treffen:

3. November, 18Uhr, wie immer im Old Murphy's.

Wenn jemand möchte BTC mit Bargeld kaufen: Ich werde 10 papieren "bitcoin tips" mit wert 0.06174226 BTC das Ich werde verkaufen fur 10 Euros pro Stück, ausser das den Preis wechselt sehr viel in den nächste Tagen. Die "Tips" komme aus https://www.bctip.org/ und sind ein einfach Weg zu tauschen ohne ein handy das Bösen benutzt uns zu verfolgen. Naturlich, wenn du trotzdem ein Smart Phone mit bringst, kannst du die Tips einlösen an die Treffen.

Wie fruher, wenn jemand ihr "tab" mit BTC, LTC oder FTC bezahlen mochten, ich werde addressen bringen. Du kannst mir mit crypto bezahlen, und ich werde Old Murphy's Euros bezahlen. Na ja, bis ein realistische Grenze. Ich werde die Preisen von cryptos Sonntag anschauen.

Bis dann!
================================
Next Meeting:

November 3, 6pm, as always at Old Murphy's.

For those looking to buy BTC with cash: I plan to bring 10 paper "bitcoin tips" of 0.06174226 BTC each that I will sell to interested parties for 10 Euros each, unless the price changes a lot in the next few days. The tips are from https://www.bctip.org/ and are a convenient way to trade without relying on one of those phones that track you everywhere. Of course, if you have a smart phone with you, then you can redeem them while at the meeting.

Again, if someone wants to pay their tab in BTC, LTC or FTC, I will bring addresses. You can pay me in crypto, and I'll pay Old Murphy's in Euros. Within reasonable limits, of course. I'll look up the prices of the various cryptocurrencies on Sunday.

Until then!

Update (Oct/Nov 2014): My github account was deleted for some reason, so the code below is no longer available there.
I found an alternative host for the code.
This is a good reminder that github.com is a third party site and there is no reason to believe they will not delete your repo.
----------------------------------------------------------------------
I've written a basic implementation of BIP0032 (HD Wallets) in Steel Bank Common Lisp (SBCL) and published it on github for anyone interested:

https://github.com/kronarev/bip0032sbcl
https://mathgate.info/bip0032sbcl/bip0032sbcl.tgz

There's no real user interface, but the basic functions are there (e.g., for private and public child key derivations). A file unittests.lisp demonstrates how to call the function using the test vectors given with BIP0032.

https://en.bitcoin.it/wiki/BIP_0032_TestVectors

It is only intended to work under Unix-based operating systems as the hash functions are performed by openssl, sha256sum and sha512sum.

I wrote this mostly to help me understand some of the technical details underlying bitcoin. I did it in Lisp because I like Lisp and because Lisp uses arbitrary-precision arithmetic.

It might be useful for people who want to generate more unit tests for HD Wallets.

In addition to computing addresses and WIF private keys for BTC, there is also code for computing addresses and WIF private keys for LTC and FTC.

I welcome feedback, even if (especially if?) it's to point out a mistake I've made in the implementation.

If anyone at the Saarland bitcoin Stammtisch this evening wants to pay their
tab in BTC, I can pay in Euros and you can pay me in BTC. I will bring some BTC
addresses and write down the amount of BTC that should be sent to the address.
I will trust that you will send it later, so you don't need to bring a mobile device.
This offer is valid until I have spent 100 Euros. 

To keep things simple, I will use an exchange rate of 1 BTC = 100 Euros.
So, if your bill is 8 Euros, I will pay the 8 Euros and you will send me .08 BTC later.

I will also bring a few LTC and FTC addresses in case someone wants to pay with those.
Simple close-enough exchange rates for these are 1 LTC = 2 Euros and 1 FTC = .10 Euros.
So, 8 Euros would be 4 LTC and 8 Euros would be 80 FTC.

---

Und jetzt versuche ich auf Deutsch.  Bitte verzeihen fehlern.

Wenn jemand an die Stammtisch heute abend die Rechnung mit BTC bezahlen möchten,
ich kann mit Euros bezahlen und du* kannst mich später mit BTC bezahlen. Ich werde
BTC addresses mitbringen, und kann schreiben wieviel zu diese address geschickt sollen.
Ich werde vertrauen das du die BTC später schicken werden.
Die angebot geht bis ich habe 100 Euros ausgegeben. 

Lass uns sagen 1 BTC = 100 Euros.
Also, wenn die rechnung ist 8 Euros, Ich bezahle die 8 Euros und du schickst mir .08BTC spater.

Ich bringe auch LTC und FTC addresses für den Fall wenn jemand so bezahlen möchten.
Lass uns sagen 1 LTC = 2 Euros and 1 FTC = .10 Euros.
Also 8 Euros ist 4 LTC und 8 Euros ist 80 FTC.

* Wenn "du" sollst "Sie" sein, bitte jedes "du" nach "Sie" und verb formen wechseln. 

The first Sunday of the month is a few days away. I assume the Saarbrücken Bitcoin Stammtisch is still on. I plan to show up at Old Murphy's at 6pm.

Ich kann auch auf Deutsch schreiben, aber vielleicht mit ein "Paare" fehler:

Ich nehme mich an das die Saarbrücken Bitcoin Stammtisch werde statt finden. Old Murphy's 18:00, wie Juli? Ich werde da sein.

I plan to come. I'm looking forward to meeting some people interested in bitcoin.