I stumbled across this site yesterday when looking for some FireFox plugins and i have to admit i can see me getting a lot of use out of it. The basic premise is pretty simple:
BugMeNot.com was created as a mechanism to quickly bypass the login of web sites that require compulsory registration and/or the collection of personal/demographic information (such as the New York Times).
Update: The people behind BugMeNot have included a link to my version of the plugin on the FAQ page. Very cool.