Check out this website, with a self-explanatory name:

Yep, login is not a verb. You don’t login. You don’t login on GMail. You don’t login to GMail. You don’t login. Period.

Link via John Gruber.

Update: Mandatory Disclaimer: You’re welcome to take it if you want, or leave it. I’d rather like this to not descend into the larger debate on correctness of language or its necessity. We’ve been there in older posts.