I was digging around the Gmail docs just to see what new stuff they've been throwing in there and came across something useful that I hadn't come across before.
A lot of people I know have multiple mail accounts some of which they use when signing up for stuff just in case for whatever reason that address gets "accidentally" sold to an organisation that will spam you silly. What Gmail does is provide a feature called "plus-addressing", which allows you to insert useful but ignored stuff into your Gmail address.
As an example:
If your mail address was firstname.lastname@example.org (Apologies to Myna Me if you have an account) and you were signing up for something at "dodgysite.com" then you could supply the mail address in the subscription form as:
The mails will still be routed to your account but if you start to get spammed and that address is being used then you have an idea of where they got your address from.
It's quite a nice feature and when used with filters can also be used to manage mail too so you could possibly give plus-addressed addresses to everyone and include some identifiable word in the address.
One thing to be aware of is that at some point if people start using this a lot then the spammers will start to circumvent it by striping out the +x part, but that doesn't seem to be the case at the moment.
EDIT: Nic points out quite rightly that this isn't strictly a Gmail feature but is part of the mail specs such as RFC5233 and yes, it is dependent on the site you're supplying the address to actually accepting a "+" (the latest version of Fring on iPhone doesn't even allow "."!!!!)