One of the few problems I have had after switching to Mac is finding a dead simple Paint replacement app. There are dozens out there for Mac, but most are loaded with features that I have never had a desire to use. I just found one that is about as simple as MS Paint was - I bought it, tried it out, and it works well. On sale right now for $8 - normally $20.

http://itunes.apple.com/us/app/paintr/id422813384?mt=12