OP's idea makes sense.

If you receive >50 cents for each dollar you spend on electricity, and you strike a deal with someone willing to pay 50% of the cost... then my sir... you have a lucrative business.

Same here in Europe. When it's cold and I anyway would need electric heating, I turn on miners. For every 1 EUR spent on electricity I get back around 50 cents.

I downloaded the app but I have no contacts 

How do I search for contacts?
Feel free to add me. I'm ezetler

Conuterparty betting/smart contracts.

I've generated some vanity addresses with vanitygen
but I don't know how to print them as QR codes.

Is there any offline (e.g. javascript) way of doing this?

The best I could find was Bitaddress, but it only lets you import one key. I'd prefer to have 5-7 addresses on one sheet.

May you recommend eBooks about Bitcoin?

Free eBooks are particularly interesting.

Here's a list of Bitcoin Brainwallets and Paper Wallets.

• https://www.bitaddress.org
• https://brainwallet.github.io/
• https://keybase.io/warp
• http://ms-brainwallet.org/

Do you know any others?

Paper wallet, smartphone wallet ...

Sometimes I want to transfer from PC to such a wallet. Now I need to write the entire address - would be great to write only the first few characters...

Is there a way to look up any address online by typing the first few characters?

Say I want to find 1ESvmL4k3wHsthoADGD5EQXmyaCCHnYQei; it would be great to type in only 1ESvmL and then get the entire address.

Obviously such a service would only work for addresses that have at least one transaction.

+1 to Foxpup. 

What is the probability that the next block will be found within 10 minutes from now?

Or within 1 minute?
Or 10 seconds?
Or 1 second?

You can mine testnet Bitcoins. They are supposed to be worthless though.

A year ago there was talk about a side-chain, Zerocoin (?) - what happened to it?
There's been a lot of hyping of a a Dark Wallet - but not much lately. Are the devs on the right track or has the project been abandoned?

And there's of course the altcoins. Darkcoin, Kryptonite, Monero - do any of these really have anything to offer, or are they just premine pump-n-dump?

Should we just conclude that Bitcoin is pseudonymous - and accept it for what it is 

I have a Monero wallet with private key, but I do not want to wait for bitmonerod to sync.

Is there any web-wallet or similar where I can import the private key?

Hmm, I still don't really get it.   
Is there a website where I can test it out?

I want to generate a multisig address. Alice, Bob and I shall have one key each.
How can I give keys to Alice and Bob and prove that I have not seen their keys?

Both Counterparty and Dogeparty were created through proof-of-burn. The developers did not raise any money. Rather they had to participate in the burn like everyone else.
I don't know Clearinghouse. May someone explain how they did it?

Are you sure the XCP got transferred? I looked at blockscan, and there is no address with 5500XCP, nor has any addresses with roughly that amount been newly deposited.
There's a slight chance you were  lucky - exchanges sometimes screw up withdrawals. Login to the exchange and see. If there was error, contact them immediately and tell them to cancel the withdrawal.

If you need Testnet coins, you can mine some or get a few from a faucet.

If you require a larger amount, you can buy from me for BTC 0.0001 each.

PM me if interested.

Why would heat from a Bitcoin miner be dangerous. My undervolted S1s run at only 40C. That's less than laptops, playstations, etc. I've never heard any report that heat from computers can be dangerous.