If you're using Spaces, you probably know that F8 will show an "Exposé-like" view of all the windows in all your spaces.
What you may not know, however, is that while in this mode, you can actually invoke Exposé's All Windows mode (by pressing F9) while the F8 Spaces mode is active. Now you can not only see all your spaces, but you'll have a clear view of each window within each space.
[robg adds: You can also drag windows from space to space while in All Windows mode. The Application Windows (F10) and Desktop (F11) modes work, too, but they exit the Spaces view when they do -- F10 and F11 will be applied to whichever space was active (highlighted) in the Spaces view.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071026000006326