Gmail provides a bunch of great ways to filter your e-mail. One of the most accurate is to append a string to the end of your user name (i.e. firstname.lastname@example.org) when signing up for a service. Gmail strips everything after the + when determining where to deliver a message, however it will still appear in the "Delivered-To" message header. You can then run a search or create a filter by searching for "deliveredto:email@example.com". This allows you to set up auto labeling or other behaviors based on the origin of the e-mail, without having to know where the message might be coming "From:".
This is also useful for setting up forwarding between Gmail accounts. Using this guarantees you can label forwarded e-mail because Delivered-To will always be the address you specified in the forwarding set up, even if the original e-mail was addressed To: a mailing list or you were originally BCCed.
There's one big problem with this set up. There are a lot of... misguided developers out there who set up their registration forms with bad e-mail validation. I'd say about 50% of the time I can't use this method because + is not a valid character in the form.
I wish people would bother to follow standards. Very annoying.