Vim is VI-iMproved: it's a souped-up version of the classic Unix editor vi. Vim is written in C, and has been ported to almost all Unix/Linux/BSD variants as well as Win/Mac OS's. Vim can run in terminal mode or in its own window, using whatever native windowing system you have (Win32, Quartz, Qt, Gtk). A kickass feature of vim is syntax highlighting.
Vim is the most productive text editor available, once you learn the cryptic vi commands that is.