Just came across this PC World article on Gmail. Here is an interesting part:

Filter messages with custom addresses and aliases:

Another convenient Gmail feature is its ability to use aliases. You can filter your incoming mail by adding a plus sign (+) and any extra characters (no spaces) to your address. For example, when you register with the Frostbite Falls Tribunesite, submit an address similar to 'scott+tribune@gmail.com'. Or provide your boss with an address like 'scott+deadline@gmail.com'. Now, you can create filters that will label, star, archive (skipping theinbox), or forward only those incoming messages. And should the cash-starved folks at the Tribune sell your e-mail address to marketers, you can change your filter to redirect messages addressed to 'scott+tribune@gmail.com' to your Spam folder. (Note that 'scott@gmail.com' isn't a legitimate address because Gmail requires that user names have at least six characters.)