From Northern Softworks:
Renicer is a GUI tool that automatically sets priority values for applications. Designed to run in the background, it automatically prioritizes whatever application is in front. Additionally, it ups the priority of the Finder, Dock, and Window Server. It also maintains a list of applications that are to be set to low priority. Applications are reniced on the fly as they are launched and brought forward. Renicer is designed to be simple enough for novices, yet flexible enough for the power user.