Android: If you've ever wished you could skip music tracks or scroll up and down with your phone's volume buttons, free app ButtonRemapper will let you reassign those functions on your rooted Android phone.
If you find there are some buttons on your phone that you just don't use (search button anyone?), you can make them more useful by remapping them to other actions with ButtonRemapper. Right now, it's a pretty simple app, letting you remap any hardware or capacitive buttons on your phone to a number of different functions. It can't assign them to open up apps at the moment, but the app is still in beta and the developers are working on adding that feature soon. For now, though, folks that want to remap either the search or camera buttons to apps can use free app Button Shortcut for the time being.
Note that this is one of those apps that can mess with your phone if not used carefully, so make a backup through ROM Manager or your recovery module before you start messing with the button mappings. If you have any problems, ButtonRemapper has a nice feature where you can return the buttons to normal upon rebooting your phone, which is nice if you have minor problems. While it works on all phones except the Galaxy S, different phones allow for a different number of remappings (the Droid, for example, only allows for two) — so if you try it out for your phone, let us know what it offers for that model in the comments.
ButtonRemapper is a free download for all non-Galaxy S Android devices. You can buy a $1.06 version in the Market, or grab the free version from the XDA-Developers forums at the link below (a free account at the forums is required).