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, Motorola Droid and Nexus One with landscape support.  For ease of use, coolbho3000 has included an autodetect option where SetCPU selects the relevant device.

SetCPU lets the user 'overclock' to the max CPU speed of their device, and underclock when the phone is idle in order to conserve battery and optimise performance.  The 'ondemand' CPU governor scales the speed between the minimum and maximum speeds depending on current CPU load, whilst the 'performance' CPU governor runs the device at max speed CPU speed - but be warned, this can drain your battery very quickly.

SetCPU also contains a few basic profiles for setting the CPU speed including for when your phone is charging or sleeping, a battery temperature profile - SetCPU can set a certain speed when the battery goes above a user-defined threshold and a battery level profile - once the battery level drops below a certain point, the SetCPU will drop the CPU speed. Profiles work by running a low resource service in the background which is disabled when the profiles are turned off.

The widget for SetCPU shows the min/max cpu speed range set (currently does not show the CPU speed in real time).  Simple themes are available in the application.  There is also an option to 'Set on Boot'.

Advanced users can also define custom frequency lists for custom kernels that SetCPU does not support or for devices with processors not listed in SetCPU's device selection screen.

Future developments include optional profile notifications in the pull down shade.

The application is available for free for XDA members, and for those who wish to donate for coolbho3000's work SetCPU is listed at USD0.99 on the Market.

To download or for more information, check out the application thread .