I've looked for ways to utilize that space in the past, the best non modding solutions are trackpads or trackballs. The problem is that they both suffer from ergonomic problems (for me at least), the hand is in an unnatural place (front row center) for critical mousing activities, which can really cause injuries. The best solution would probably be to use a programmable/remappable keypad, maybe one of the XKeys, but I haven't looked into that.
Given that disclaimer, you can get usually get trackpads or slim trackballs to work by attaching tallish rubber feet to the bottom (3 ideally) to balance in the center. Even a large trackpad like the Apple can be made to work. The best trackball is the Marble mouse, which fits perfectly there (or almost) with three rubber feet. Again I didn't find this too ergonomic, even though it seemed perfect.
I'd be really tempted to try a numpad but I've got other things brewing.