FreeRAM XP Pro
The program optimizes how apps use memory, forcing them to use the minimum required and leaving as much as possible free for other programs. This should (in theory) make your computer faster and more responsive. Although it's tough to tell how good a job it does, it seemed to be more intelligent than Windows in keeping memory free. In addition, the ability to free up large chunks of memory is useful if you are going to be running a program that needs it (a feature FreeRAM XP Pro calls AutoFree).
Of course, this utility won't make your system as fast as adding more memory to your motherboard, but it's a useful tool if you find yourself running low on memory and want to try and make more efficient use of what you have. FreeRAM XP Pro is a little easier to use than RAMBooster, and the AutoFree mode is easier to use than the equivalent mode in RAMBooster.
However, bear in mind that the developers don't provide direct support for this free program. As a result, there is no telephone or e-mail support, and the feedback link on their Web site didn't work when I tried it.