Android: If you like to overclock or enable other certain features on your device, you'll need to use a custom Android kernel. Kernel Manager makes installing third-party kernels on your rooted phone an easy, one-tap process.
By default, certain Android kernels don't allow you to use all possible features of the device. If you're looking to overclock your phone beyond a certain point, underclock it so it uses less power, or enable advanced features like Compcache, you'll need to flash another kernel that has those features enabled.
If you've used previously mentioned ROM Manager to find and flash custom ROMs to your device, Kernel Manager will seem pretty familiar. Kernel Manager makes the process easy by providing you with a list of available kernels for your device and letting you flash them right then and there. Just filter the list by what features you want, find the best one for your needs, hit "More Info", and scroll down to download and flash the kernel to your device. You can even rate them on their stability and features for other users.
The app is brand new, so it doesn't support every phone quite yet (at least at the time of this writing), but for an app that's only a few days old, they've already got quite a few supported phones in the database. Head to the app's Market page to see a list of supported devices, or download it to your phone and keep an eye on it to see when kernels are released for your phone.
Kernel Manager is a $3.98 download for rooted Android phones. There's also a free "lite" version that allows you to see what kernels are available (which of course you could then seek out online and flash yourself using ROM Manager or your phone's recovery mode).