Gmail has an interesting quirk where you can add a plus sign (+) after your Gmail address, and it'll still get to your inbox. It's called plus-addressing, and it essentially gives you an unlimited number of e-mail addresses to play with. Here's how it works: say your address is firstname.lastname@example.org, and you want to automatically label all work e-mails. Add a plus sign and a phrase to make it email@example.com and set up a filter to label it work (to access your filters go to Settings->Filters and create a filter for messages addressed to firstname.lastname@example.org. Then add the label work).
More real world examples:
Find out who is spamming you: Be sure to use plus-addressing for every form you fill out online and give each site a different plus address.
Example: You could use
email@example.com for nytimes.com
firstname.lastname@example.org for freestuff.com
Then you can tell which site has given your e-mail address to spammers, and automatically send them to the trash.
Automatically label your incoming mail: I've talked about that above.
Archive your mail: If you receive periodic updates about your bank account balance or are subscribed to a lot of mailing lists that you don't check often, then you can send that sort of mail to the archives and bypass your Inbox.
Example: For the mailing list, you could give email@example.com as your address, and assign a filter that will archive mail to that address automatically. Then you can just check in once in a while on the archive if you want to catch up.