Removing hidden and damaged webparts from a page

You can remove hidden or damaged webparts from a page by appending "?contents=1" to the url (if the url already contains a querystring parameter replace "?" with "&"). This will bring up a screen listing all the webparts on the page (both visible and hidden). You can then select any web part and remove it.

If a webpart is causing significant damaged to a page (ie throwing an exception which isn't handled in the webpart) then you should see a SharePoint error page with a link to the remove webparts from the current page.