What are some Textmate alternatives that have the completion key feature?
The Cocoa build of Emacs 23 is very good and has a ton of extensions and virtually limitless scope for customisation.

The learning curve can be a bit steep, but it's well worth the investment in time once you get over the initial hump.

If you are comfortable with TextMate, the Terminal and these sorts of things, you will be fine, http://emacswiki.org is a great resource.