Is there a free, lightweight text editor that isn't text-only but also isn't a heavyweight IDE?

Solution 1:

Have you considered Xcode?

I know you said you don't like TextMate, but it's one that meet pretty much all your needs.

which is fast

TextMate
Really fast

Xcode
Not that fast but not Eclipse/Netbeans-slow

which does syntax highlighting

TextMate
Very complete and extensible

Xcode
Does syntax highlighting, it depends on your needs

were I can define own syntaxes (or there are many plugins e.g. for YAML),

TextMate
Very, very extensible

Xcode
Syntax extensions are possible

where the developers are active,

TextMate
Some say it's vaporware, yet very few other text editor can compete with its current features

Xcode
Apple development of this program is very active

which does not have the look & feeling of TextWrangler

TextMate
If you don't like the sliding bar, there's a plugin to replace it by a drawer

Xcode
Doesn't have a sliding bar

which can define projects (like Coda can)

TextMate / Xcode
Do projects

which includes a basic support for Git and/or SVN

TextMate
Support for Git/SVN/Mercurial (and probaby more) via built-in and added plugins

Xcode
SVN support for 3.x, added Git in Xcode 4

which supports a fast autocompletion

TextMate
Basic variable/function completion
very complete code completion via bundles

Xcode
Basic variable/function completion

MacOS X 10.6 compatible

TextMate
Some compatibility issue

Xcode
Fully compatible

Solution 2:

I would say that MacVim but it's of course a more complicated tool based on the traditional unix vim text-editor with a steep learning curve.

By the way, which aspect of TextMate didn't appeal you?

Solution 3:

I think that Aquamacs might do the trick!

Solution 4:

Sublime Text 2

Support for TextMate snippets, languages and color schemes. Split view, tabbing, projects, commands. Full-screen on Lion and a "focused" mode. Extensible, too.

Very fast, great multiple selection mode and cross-platform.
Currently priced at $59, but a free version is available with occasional alerts.

It also has a rapidly-growing plugin API and surrounding community.

Check it out, it's freaking awesome.

Update: Sublime Text 2 is now superseded by Sublime Text 3 which features various enhancement and performance improvements.