Every now and then you stumble upon a nugget of pure gold on the internet. This is a great little bit of info which I found out about in a Slashdot comment.
The trick? This little character here --> +
If I'd like to give out an email address and use it to filter on (for example) I can give out
email@example.com and it will be delivered to my inbox. In fact, I can put anything after the "+" that is allowed in a normal email address. See? Pretty darn cool isn't it?