The Yahoo Toolbar for Firefox (beta) is released! This is handy for me since I use Yahoo Mail, Calendar, AddressBook and Notepad a lot. Two features I liked are 'Search only the current site' and the bookmarks integration.
The only downside is that it takes up a chunk of screen estate (of course).
On an unrelated note, check out the new Yahoo! Music logo - me liiiike! :)
Note: The toolbar page says 'Requires Firefox 1.0 for Windows' but its working fine for me on Linux. The beta release notes has more info.
Update: I just noticed that Google has a IE-only toolbar.