Is it possible to add a miner's fee after you send your bitcoins? I accidentally sent a transaction without a miner's fee and I need it to clear as soon as possible. But, as far as I can tell it's going to take days if it goes through at all. Is there any way that I can pay a fee on top of what I've already sent?
Rebroadcasting a similar transaction with a fee attached will invalidate the one that's waiting. Basically you initiate a double-spend and the new one (with a fee) will get confirmed.  Once it's confirmed, the old transaction will be invalid and forgotten.

Your client probably does not allow double-spend attempts, but it's possible via blockchain.info. Warning: Don't create a raw transaction unless you really know what you are doing!

People are working on making fee changes easier for us mere mortals; one example is/was (seems to be defunct) ConfirmNext.