Don't know about you but a lot of what I do requires having access to multiple email addresses (testing websites that require signup etc). To aide my development I have a few email accounts I use - a Gmail, a Hotmail, a Yahoo and a Google Apps one (my main account). That's only four addresses though, which can be quite limiting at times.
In steps Gmail. If you have a Gmail account then you have access to "infinite" email addresses using that one account.
The Gmail account I use for general testing is:
email@example.com (I left a t off on purpose)
But I could also use any of the following:
firstname.lastname@example.org, etc etc
You get the idea. You can tack a + sign and then any random string after it to invent a new email address. Obviously it still delivers to your Gmail account and it lets you signup to a website over and over as a "different" user.