Cooliris was pointed out in another thread. But I found it to be too intrusive.
However, I do find these two useful:
Google Browser Sync - keeps my work and home bookmarks in sync. There are other synchronizer plugins out there, but I already had the google id from having gmail.
IE Tab - There are some sites that are strictly tailored for Internet Explorer. But I prefer Firefox in all other things. This plugin uses the IE component to embed an ie browser in your Firefox tab. Means I don't ever have to use IE now.
Any other useful ones out there?