SketchBook Mobile Express is pretty much the same as the full version, just less layers.
Sketch n Draw and iSketcher are pretty good too. Here are some on the Kindle HD that are rated 3 stars and up:
http://www.amazon.com/s/ref=sr_nr_p_...nid=2479574011
I'm pretty sure in the next month or so, they'll make one of the nice ones free for FAOTD. You might want to keep an eye on those BTW.