My recent post on Rybka's prowess (see Rybka 1 - Fischer / Huebner / Kasparov 0), combined with several years of complacency in the face of ever more powerful chess engines, convinced me to upgrade my analytical workbench. After conducting a little research, I decided to go with the Arena GUI coupled with Rybka 2.2. The package can be found in the Downloads area of the Arena 2.0.1 Chess GUI. It's free.
I easily installed the software, along with a few other engines distributed with Arena, and configured the GUI to match the working method that I currently use. Adapting tools to work the way you do is more important than adapting the way you work to fit the tools. The software might be free, but the time spent learning how to use it isn't.