While this is Windows only it has the benefit over using Kavoom / Multiplicity instead of Synergy of being free.
  • Works with mixed Vista(X64)/XP set ups / multi-monitor per computer setups / vertical / rows / columns monitor arrangements.
  • There's a button in the control window to switch to broadcasting keystrokes/mouse clicks to all windows (although you'll have to bind a key to turn this off again).
  • Keys can be bound to turn on/off screen edge mouse transitions.
  • You can record macros and assign them to hotkeys in the program (WoW ToS permitting).
  • The Clipboard is shared between all computers.
  • Supports login screen use on the client computers.
To be honest I'm surprised so few people are using it, as combined with it's zero cost and ease of use I see no reason to even consider any other alternatives.

http://www.inputdirector.com/index.html


[edit] And no, I'm not involved in any way with the project, it's just the first thing I found that did the job well enough, when I went looking for a Synergy alternative.