I know it exists, but every time I need it I still spend 15 minutes searching. So, once and for all,  here it is: ?contents=1.  Appending that to the URL of a web part page in WSS or SPS will display a list of web parts on the page with options to close or delete one or more of the webparts.

Next time you can't open your site because a webpart is breaking the page, or for any other reason (my current error is 'The page you selected contains a list that does not exist..") then remember: contents=1.