Very simply what software KVM's would you recommend for using on Windows?
http://synergy2.sourceforge.net with some success.
It is cross platform, lets you move your mouse off the edge of one screen and onto the next screen even across systems.
As mentioned, synergy works very well if you have all the screens near each other and just want to use a single keyboard and mouse between them. It is also cross-platform. I use it with my linux desktop with my windows laptop sitting just to the right on my desk and it works flawlessly.
For more like a remote desktop kind of solution, there is of course VNC, of which there are several versions around for different operating systems. I tend to use either UltraVNC or TightVNC on windows. (UltraVNC has a nice single-click exe builder that makes an ~200K exe which when run will install a temporary "server", and connect to a predetermined IP address, to the "client" that is in listening mode. Very handy when you're trying to help somebody over the phone.)
If you are specifically needing to access Unix boxes remotely, you might want to look at NX. It is very fast, even on reasonably slow connections.