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 email@example.com, and you want to automatically label all work e-mails. Add a plus sign and a phrase to make it firstname.lastname@example.org 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 email@example.com. Then add the label work).
You can use this small trick to create web site specific addresses and set up filters for them to catch companies who spam or sell email addresses of its customers.
Let's say you need to sign up for a mailing list that interests you, but you're afraid spammers might get your address. We'll call the list "exoticflowers". Sign up with the list using the address "firstname.lastname@example.org". Email to that address will still come to your "email@example.com" address even though the "To:" will include that "+exoticflowers" in it.
Then set up Gmail filters to shuttle that +exoticflowers email past the inbox to a specific label.
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
firstname.lastname@example.org for nytimes.com
email@example.com for freestuff.com
Then you can tell which site has given your e-mail address to spammers, and automatically send them to the trash.
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 firstname.lastname@example.org 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.
Also, You can reformat your existing Gmail address by inserting dots (periods) anywhere within your Gmail username. The Gmail Help pages explain:
Because Gmail doesn't recognize dots as characters within usernames, adding or removing dots from a Gmail address won't change the actual destination address. Messages sent to email@example.com and firstname.lastname@example.org are all delivered to your Inbox, and only yours.