Android 4.0+ (rooted): One of the best things about having a rooted phone is that you can really get your hands dirty tweaking it. eXperience is a new app that lets you overclock your phone's CPU, give you independent volume controls, and control music with the volume buttons.
eXperience's biggest benefit is that it can make these system-level changes independent of your phone's kernel. The app lets you enable "touchwake", meaning that when your phone goes to sleep and the screen turns off, you can tap the screen instead of pressing the power button to wake it right back up. The app also gives you the ability to control music playback with your phone's volume keys, and lets you overclock (or underclock) your phone's CPU with a few taps. It also lets you adjust the volume of your headset, microphone, attached microphone and built-in speaker individually, a feature I've been missing for a long time.
Technically, eXperience is only available for a select group of devices (check the list for yours at Google Play) but it actually works on more phones than the ones listed. To boot, some of the features in the app don't work on all devices. If your phone isn't supported, try the free "test" version, which can collect the system data required to make the app compatible and email it to the dev so he can include your device in the next build.
The "pro" version of the app will set you back $2.99 at Google Play, but if you're not sure it will work on your device, you can try the free "Test" version first.