I've tried a number of text editors for OS X, including Smultron, Taco Edit, Aquamacs, and the built-in TextEdit. All of them are pretty good, but TextWrangler has made them all obsolete on my machine. This is an astoundingly full-featured program with uses in many different environments. I have several different requirements for a text editor, including CSS editing, HTML blog posts, and writing command and dictionary files for Stata (a statistics package). I used to use different programs for each: Aquamacs for statistics, Smultron for CSS, Taco for HTML things. Textwrangler now handles all of those duties.

Compared to other free editors, Textwrangler offers much more in the way of preferences, options, keyboard shortcuts and menus. It also has good help, and an excellent built-in tutorial. These are the sorts of things that don't always make it into one-person open-source software programs, and as much as I love those freeware endeavors this program just has a fuller feature set. As far as performance, it's very light on the memory, using barely more than finder in my 10.4 PPC setup.

My summary is: all the benefits of commercial software at the freeware price. I really can't believe how good this program is, no complaints so far.