Google lets you put a + sign in you email address that it ignores but you can use to create filters. For example, when you create an account at buy.com, just I just tell them my email address is BenShoemate+buy.com@gmail.com. If I start getting ads for "V I A GR A" I know who to boycott, report, complain, filter. Cool.
Also, gmail ignores the . in your name. So even if you signed up for FirstLast@gmail.com you can tell people at work its First.Last@gmail.com and set up a filter to auto tag those as work. Also cool.