This application lets you control Lego Mindstorms NXT robots from Android devices via Bluetooth.
It's available for free on Google Play Store. A video of the application in action:
And two screenshots:
Feel free to contact me at jfedor@jfedor.org if you have any questions or comments.
If you're interested in the USB/Bluetooth protocol used to control the NXT brick, you can download the documentation from the Lego Mindstorms site. By the way, Lego's own Android application, MINDdroid, is also open source