Speed Freak (Free).

From the Developer's website:

Speed Freak, at a user-specified interval, reprioritizes applications so that the frontmost (or active) application gets more processor time than background applications. By default in Mac OS X, all processes have equal priority. By using Speed Freak, you override this default by increasing the priority of whatever application you are currently using, resulting in faster application performance. Speed Freak accomplishes this through the Unix "renice" command.

Speed Freak does not affect graphics card performance and will show little to no increase in 3D games. Although Speed Freak does not affect network performance, the increased CPU priority it offers should increase performance in internet applications. For example, web pages with complicated HTML should render faster but download speed will not change.

For more information, see the included "Read Me" document.

Get it here.