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TeamViewer, which works with Windows, Mac OS X, Linux, Ubuntu, iPhone, iPad, etc. It's very secure and quite simple to set up and use.
 
What exactly are you trying to accomplish?

If you just want to view/control the Mac desktop from a Windows machine on the same local network, just turn on Desktop Sharing in the System Preferences, and enable VNC in the advanced box - it's built into OS X; then download one of a number of free/open-source VNC clients for Windows.

If you want remote-over-the-internet access, you can either do some advanced VNC setup on routers, or use something like TeamViewer, Go2MyPC, WebEx, or the like.
 
Guess we need to know what you really want. I was recently introduced to a program called Synergy that lets me share a mouse/keyboard between different computers/OS's. Works well and I can copy/paste from one OS to the other, which is nice when I'm working on my mac but have to e-mail something on my PC.
 
I am a big team viewer fan. If there is something better no one has told me about it.
 
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