For any sysfs-enabled ROM e.g most custom ROMS
It allows you to use the highest frequency your phone supports and gain some precious FPS in your preferred game, or go down to the lowest frequency and gain some more juice when your phone is sitting idle.
All of this without any memory footprint or services running in the background!
It has been reported working on a large number of rooted  devices: HTC Dream/G1, Magic, Desire, Desire HD, Nexus One, Incredible, Hero, Desire Z/G2, EVO 4G, Samsung Galaxy S, Galaxy SII, Galaxy Tab, Nexus S, ZTE Blade, Huavei Ideos U8150 and U8109, LG Optimus One, Optimus Dual/G2X, Optimus Black, Motorola Flipout, Droid 2 and Milestone, Acer Liquid A1, Sony Xperia X8, X10, W8, even Advent Vega, Archos 43 IT and Nook Color!
Is your device supported? You need ROOT permissions and you'll probably need to be running a CUSTOM ROM (an example is the famous CyanogenMod) with kernel frequency scaling enabled. If you are unsure... just install the app and see for yourself!
- Easy GUI to set frequencies, governor and i/o scheduler
- "safety valve" to avoid boot-loops in case of unsafe settings (more info in the homepage)
- Stats screen to see the effective usage of frequencies
key:cpu, controller, cpu control, cpu master, cpu controller