SelectThing
The extension SelectThing makes adding books to your LibraryThing account convenient and unintrusive: you just select text, right-click, and choose "Search on LibraryThing" from the menu. I swear it's even easier than that description makes it sound, just see it in action.
Current Version: 1.0 for Firefox 1.0, released 2005-11-20
Future Releases:
No releases are currently planned. SelectThing needs a new maintainer to update it to Firefox 1.5 and 2.0. Please contact Peter Harkins if you're interested in taking over the project.