Freeware - Remote Control

LogMeIn.com - http://www.logmein.com/

LogMeIn Free gives you the flexibility to access and control your PCs or Macs from anywhere. It's safe, easy and completely free.
This is an excellent remote control solution to access any computer, anywhere. (Requires you to setup an account on their site, but is easy and free - no credit card required).

TightVNC - http://www.tightvnc.com/

TightVNC is a free remote control software package. With TightVNC, you can see the desktop of a remote machine and control it with your local mouse and keyboard, just like you would do it sitting in the front of that computer. TightVNC is:
* free for both personal and commercial usage, with full source code available,
* useful in administration, tech support, education, and for many other purposes,
* cross-platform, available for Windows and Unix, with Java client included,
* compatible with standard VNC software, conforming to RFB protocol specifications.

This program is good if your computers are on a network. Note that Windows has a program built into it that is very similar (Remote Desktop Connection / Terminal Services Client - mstsc.exe)