I made a little hovering project since this is a common Unity3D question.
It has both a simple cube that hovers, but also features a driveable hovercraft.
It all uses the Unity3D physics system. Click image to play it now!
http://plbm.com/hovering/index.html
And here is the full project including all source code:
https://www.bitbucket.org/kurtdekker
or
https://www.github.com/kurtdekker
Leave a comment if you have any questions!