Turbo Tray is a program that puts an icon in the System Tray and lets you single-click on it to change the processor's frequency, shifting through several values.
The program is memory-resident and will retain its settings even when the Pocket PC is turned off (but it won't survive a Soft Reset, of course).
Originally designed to let possessors of an IPAQ 1910 gain access to a frequency of 300 MHz, thereby obtaining better performance from video programs and certain games, it was subsequently improved, and on the author's site you'll find several versions (Turbo Tray 472, Turbo Tray 498, Turbo Tray 333), adapted to various machines. Vice versa, you can also reduce processor frequency to increase battery life.
Used in conjunction with Speed Stepper by the same author, here's a way to exploit low-end machines to best advantage. But it's not without its risks.