Apple - 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.

Sublime Text 2 is proceeding nicely. Reminds me a lot of Textmate

Overview: http://www.sublimetext.com/blog/articles/sublime-text-2-public-alpha

Download for ver. 2: http://www.sublimetext.com/2

One option is Smultron, which is available from the MacApp Store for 4,99. It offers exactly what you need (really good autocompletion). You find more information on it in the first comment on this post. There is also a free fork of it named Fraise. I didn't try Fraise yet, but it seems to be as good as Smultron.

Another option you have is Vim. But getting to Vim is not easy. If you are coming from TextMate I would recommend MacVim in combination with Janus. It is developed by Ex-TextMate guys that came to Vim and they did a really good job to provide some functionality that TextMate Users love. The Janus Page is btw. a good starting point for a Vim Transition.