If you have two windows based computers where you want to control both systems with the same keyboard, I would highly recommend the Input Director freeware over Synergy. Input director has a whole slew of advantages over synergy:
1. The mouse seamlessly slides from desktop to another without the use of hotkeys.
2. Special mouse/keyboard functions for my logitech mouse such as the document flip not only work on the host but the client as well.
3. Input director doesn’t cause the mouse input to flip out when a Direct X application is open (unlike synergy).
4. Support for both computers (master and slave) having multiple monitors.
The only disadvantage about Input Director is that it works for only windows based OS’s (unlike synergy which works for multiple OS’s – ie Linux, Mac, etc.). If you are controlling multiple computers which are all running windows OS’s, I recommend Input Director over Synergy. Also, Input director had an update this past September whereas Synergy hasn’t been updated in years…I have tested both and got both working but I can only vouch for Synergy’s functionality on two windows machines.