Vim is a popular vi clone. Although I often feel more minimalistic and tend to prefere nvi on the console, I used Vim as the main GUI editor on my Macs. You can get a precompiled version of Vim for Mac OS X on macvim.org.
I have written a number of patches for Vim. Since the interesting stuff already became part of the official version of Vim, and the remaining stuff is far outdated, I don’t offer them here any more. Sorry.