Just click to install
DS Forums
DS Forums (Google)*
Digital Spy
eBuyer.com
GTAForums.com*
IGN.com
LOVEFiLM
Multimap (UK/GB)
Play.com
TheRegisterPlugins marked with an asterisk (*) send data back to this site for processing (I use a PHP script to piece together the information, then send the user to the destination) - this is unavoidable. I don't look at what's submitted, but they are recorded in the server's log file.
Windows: Just delete the corresponding 'png' and 'src' file from the C:\Program Files\Mozilla Firefox\searchplugins folder.
OS X: Ctrl-Click the 'Firefox' application, and select 'Show Package Contents'. Navigate to Contents/MacOS/searchplugins, and delete the appropriate files.
Just open one of the 'src' files in a text editor (in the folder specified above) - it's pretty self explanitory.
Nice extention to remove the ads on Google, use Google Suggest by default (on search and result pages), point links directly to images on image search, add other places to search for images, remove the ads/related links from Gmail, force Gmail to use HTTPS (secure SSL), and a couple of other useful bits. Adapted from the CustomizeGoogle Greasemonkey script, and compiled into a standalone extention. Enjoy!