TextMate

See ​http://macromates.com/. TextMate is a very good text editor for ​Mac OS X.

There's a Markup bundle for TextMate that provides:

• Snippets for expressions and the various template directives,
• Commands for rendering templates, and checking their syntax,
• Templates for creating new Markup template files.

You can find the bundle here, and check it out from Subversion as follows:

$ svn co http://svn.edgewall.org/repos/markup/contrib/textmate/Markup Templates.tmbundle

You can then simply double-click the Markup Templates.tmbundle folder to install the bundle.