SetCPU is a useful 'on the fly' clocking tool for Android root users developed by coolbho3000. SetCPU has been around since April 2009 and has been receiving regular updates.
It will work with rooted Android devices only on 1.5 (Cupcake), 1.6 (Donut) and 2.0/2.1 (Eclair).
Supported devices include (but are not limited to) HTC Dream, Magic, Hero,...