Quite a lot of simple USB devices work on Android Tablets via the USB host.
- Common layout USB keyboards
- Most USB mice
- USB flash drives even some USB hard drives (as long as they are self-powered)
**Please be noted that for a USB device to work in Android the drivers need to be there, Google have added drivers for many devices but not for all. The best way to know if your USB device is compatible with Android is to contact the USB device's manufacturer. Or simply plug it in and see.