Tablet Remote is an Android app that allows you to manipulate an Android phone or tablet with another Android-powered device. While it's a useful app for those using their tablets as ghetto media players, the fact you can do anything with the device makes it handy for other situations as well.
The app has a special user interface designed for playing media, but you're free to do whatever you want with it. Tablet Remote communicates over Bluetooth and both devices are required to have the app installed for it to function. Setup involves making sure Bluetooth is enabled, scanning for the devices within the app and connecting them up.
Addictive Tips gave the app a go with a HTC Desire HD and Desire Z — with the HD controlling the Z — and didn't run into any problems. Response times proved to be quick.
The app is free to download, so there's no reason not to give it a spin if you're fortunate enough to own multiple Android phones and/or tablets.