ViM is Vi-Mproved (Vi Improved) It's like Vi but on steroids! Vi is great but ViM is awesome!
ViM has a Vi compatibility mode that switches off all the extras and behaves exactly like Vi. It's possible to execute the command vi and get ViM in vi compatible mode. (how it's done on RedHat and other Linux distro's) Then execute vim and get the console ViM. In addition you get a graphical Aqua ViM for OS X. Finally, it can also work in X11 as well. (you'll have to compile it yourself to get all three at once, probably having two different gVim executables; one for X11 and one for Carbon OS X)
You can install gViM, which will give you a graphical ViM. It's been ported via Carbon. The latest binary version can be found here http://macvim.swdev.org/OSX/ There's even instructions for compiling it yourself so you can include different options.
Here http://vim.sf.net/ is the main ViM homepage where you can get scripts and tips from thousands of ViM users.
ViM is orphanware meaning all they ask is that you donate some money to the Ugandan orphans by either making a donation or buying the New Riders book. You can read the book online here http://www.newriders.com/books/opl/ebooks/0735710015.html or you can download a PDF of the book here http://www.truth.sk/vim/vimbook-OPL.pdf
Of course if you prefer EMACS then you can also find an OS X Carbon port here: http://www.porkrind.org/emacs/
No need to start the vi/ViM v.s. EMACS wars on this articles comments. Try both, give both a fair shot and choose what suits you best. EMACS is more powerful than ViM. It can do a whole lot more. EMACS is the kitchen sink of tools. ViM only wishes to be an editor. EMACS is more of an evironment that can be or do whatever you can imagine.
Console EMACS is already included in OS X as GNU Emacs 21.1.1. The next major version of EMACS will include EMACS Carbon in the main source tree. The link above is for the beta of that version.