While the more popular and more cross-device SetCPU is a great overclocking (and underclocking) tool for advanced users, folks looking to just eke a bit more speed out of their rooted device might prefer DX/D2 Overclocker. Instead of having a number of maximum and minimum settings, profiles, and custom kernels (that you have to seek out, download and flash), this app condenses everything into one, simple menu with a few presets. Just open it up, tap the setting you want, and apply the settings. What's even cooler is that DX/D2 Overclocker has a stress test function, so you can see if your phone can actually handle a low voltage or high speed before putting it to regular use.
DX/D2 Overclocker is a $.99 download for the Motorola Droid X and Droid 2 only, and you need to have rooted your phone beforehand.