DBoost

With DBoost, you can improve your Mac's performance by prioritizing CPU-usage on an app-by-app basis.
This Mac OS X utility redistributes CPU time to give the frontmost application about 20% to 30% more processor time than it gets normally.

It uses the UNIX command "renice" to change the priority of the frontmost application so that the processor gives it more time. You can also manually set the priority of all applications via the "Status" window, where you also can force quit and check the priority of applications and processes.