This is a great little known feature, because you can use it to identify which websites are selling your email info to spammers. For example, lets say you want to fill out a sweepstakes form on somesite.com and it asks for your email address. Instead of telling them your address is [email protected], tell them it is [email protected] That way you know that any email that is sent to that address ([email protected]) ORIGINATED in some way from somesite.com - either the email is from them, or they sold you out and gave it to spammers and now you know who to blame. And you can now set up a filter to delete all email sent to [email protected] if it's getting spamridden, making gmail accounts have automatic disposable addresses.
This is very similar to yahoo mail's "Address Guard" feature, except that you don't have to login to gmail to create the disposable address every time you want a new one. Just add a plus sign to your username and put whatever you want after the plus sign and you've got your new address. Pretty nifty.