Firefox only (Win/Mac/Linux): Firefox extension JavaScript Options gives you finer-tuned controls on what web site scripts are allowed to do to your browser.
Mac OS X only: Inspired by previously mentioned TweetMyPC, free application TweetMyMac remote controls your Mac from anywhere you can access Twitter, allowing you to restart and shutdown your computer, take a screenshot, open a torrent and more.
A design document posted at Google’s Chromium development site suggests that a system for desktop notifications would allow Google Chrome to notify when downloads are done, web sites need one’s attention and let extension developers creatively grab attention.
