I have a Dodocase Bookback for my iPad 2 (in conjunction with a Smart Cover). It has a nice texture to it that adds a little grip. Though I haven't tried to remove it, Dodocase promises that it shouldn't leave any residue. http://www.dodocase.com/products/bookback-for-ipad2
I used the normal case initially but then I turned to the Gelaskin, as the previous was a little heavier. but unfortunately I got few marks, but they are at least better than the whole rough iPhone without the cover!
I have one of those large vinyl one (the wizard of oz ) and before that I had snow white holding the "Apple" they both cost about 13 bucks and when I removed it there was minimal adhesive on the iPad and that was easy to remove ,I just rubbed a cloth over it