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:
myname@gmail.com (I left a t off on purpose)
But I could also use any of the following:
myname+1@gmail.com
myname+2@gmail.com
myname+3@gmail.com
myname+123456789@gmail.com, 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.