Well there are a few out there, but I think that the most generic piece of software is VNC. VNC comes for lots of platforms (inclusing the most popular ones: Windows, Linux and MacOS) and it doesn't cost a thing. It's highly configurable and has quite some features (depending upon the implementation). You can get Ultr@VNC (my favourite implementation, there are several though) from http://ultravnc.sourceforge.net.
If you only need limited functionality from (modern) Windows to Windows systems, you might want to look into the remote desktop features of Windows. They are pretty easy to use, but they are quite limited compared to VNC.
If you need any help in setting this up, feel free to ask this. It's quite easy and we sure can guide you through!