You can fairly easily use a laptop, if that's interesting to you. It's actually quite easy to setup and configure. You'll install QuickSynergy (It's in Ubuntu Software Center) on both your mediacenter and your laptop and then configure the mediacenter to connect to your laptop.

If you're not familiar with it, Synergy lets you define another computer similar to having an external monitor. That is to say, you'll move the mouse pointer outside of the laptops screen and then you control the other PC with both mouse and keyboard.

One thing you should be aware of, is that the controlling PC, a laptop in this case, is called the server and that the mediacenter will be a client. Configuration is quite simple. On the mediacenter, you'll open the "Use" page in QuickSynergy and enter the laptops hostname in the server hostname field, and the mediacenter hostname in the screen name field. Append .local to the hostnames, so if your laptop is called mylaptop, then it becomes mylaptop.local. Click execute.

On the laptop, you run QuickSynergy, but this time, you open the "Share" page. Enter the mediacenter hostname in one of the four fields (again, this will be like mediacenter.local) and click Execute. You can now move the mouse pointer from the laptop and onto the mediacenter, and the keyboard and mouse will work as if you had a keyboard and mouse connected to the mediacenter.

I know it's not exactly what you asked for, but it is a completely free solution as long as you just have a laptop. :)

It works with OS X and Windows too, by the way.