Coinapult sends to any e-mail address or mobile (U.S. and Canada):
Blockchain.info also does this, to e-mail as well as to nearly any mobile anywhere:
They even will send to a friend on Facebook:
If you only have a Twitter account:
With BTCTip and Coinapult, if the recipient never claims the coin, they are returned to the sender after a period of time (like 30 days). With Blockchain.info, the sender can redeem the private key, essentially to cancel the transfer if the recipient never claims the funds or any other reason.
You can also share the private key for an address so that the recipient can redeem the bitcoins. Here's an easy way to create a paper bitcoin wallet:
Physical bitcoins can be sent to a party that doesn't know what a bitcoin is yet. Casascius is one provider of a physical bitcoins:
There are also redeemable codes from exchanges. These too can be passed to somebody who can redeem the code to obtain the value stored against it.
Now any bearer codes, like the paper wallet's private key or the redeemable codes need to be transferred securely. You wouldn't want to send a Mt. Gox redeemable code through e-mail where anyone sniffing the network between you and the recipient can spend the funds. But used properly, these function just fine for transferring bitcoins (or USDs, EURs, GBPs, etc., etc. as well, from the exchanges and wallets that offer them) from one person to the next without knowing the recipient's bitcoin address or account number, e-mail, or anything else.
Low security, instant wallets like Instawallet and Easywallet.org require no username and password -- you just need the URL.
For your specific use cases, you could send the URL as a private message (PM) to the party you wish to send funds to. Hacker News is the only one that you listed that has no PM method, but most everyone on HN using their own identity also uses some other social media which you could send the funds through.