Similar to Textmate, but cross platform and under active development, I recently stumbled across Sublime Text 2.
On a quick google search for drupal and sublime, I got this page which has a Drupal6 autocompletion file all set for use with Sublime.
http://tanc.me/blog/2011/drupal-autocomplete-file-sublime-text-2
Sublime Text 2 is still in the alpha phase, thus not feature complete by any means. Though it is stable apparently, with new releases coming out about once a week. The trial period is indefinite, though it is a paid for program (not sure as to the cost of it).
It can also use textmate bundles with some modifications. Some files can be used directly, other need to be modified. I'm skipping this for now though, since I found the auto-completion mentioned above.
