I bought mine from here:

Rubberized RUBBER HARD COVER CASE SKIN FOR NOKIA N900
http://stores.ebay.com/qcellfashionaccessory

I think they are made of clear hard plastic.
I think there is rubberized feel colored spray paint on it.

+They are light and easy to put on.
+They look and feel good.
+They do not make the N900 feel any bigger.
+/- The buttons on top and side is better protected/harder to reach.
-The one for the screen does not stick very well.

I decided to get a better Screen Protector than what I have before I eventually use a "needle head" of silicon or similar a few places between the N900 and the hardcover, this would stick it on safer.

Pictures here


.

.

.

.

.

.

.