This is as much a note to myself as anything else, but Firefox 2.0 has a great feature for people like me who tend to browse with ten gazillion tabs open. If you accidentally close one tab, or several, by mistake and you want to bring them back, you can do so by pressing Shift-Cmd-T on OS X or Shift-Ctrl-T on Windows. They'll reappear in the order you closed them.
If you close a whole window though, you can't retrieve the closed tabs in that window in another window with the key combination (though, as Riccardo points out in the comments you can reopen them from the History).
One of many little features of Firefox 2.0 that keeps it eons ahead of the annoying in so many ways IE 7.