jEdit is a mature and well-designed programmer´s text editor with 7 years of development behind it.
While jEdit beats many expensive development tools for features and ease of use, it is released as free software with full source code, provided under the terms of the GNU General Public License.

Features:

Written in Java, so it runs on Mac OS X, OS/2, Unix, VMS and Windows.
Built-in macro language; extensible plugin architecture. Dozens of macros and plugins available.
Plugins can be downloaded and installed from within jEdit using the "plugin manager" feature.
Auto indent, and syntax highlighting for more than 130 languages.
Supports a large number of character encodings including UTF8 and Unicode.
Folding for selectively hiding regions of text.
Word wrap.
Highly configurable and customizable.
Every other feature, both basic and advanced, you would expect to find in a text editor.

Editors Note: This will run on any operating system that has Sun Java installed, including Macintosh, Linux and Windows.

Download Java Runtime Environment here.