jedit

jEdit is a mature programmer's text editor with hundreds (counting the time developing plugins) of person-years of development behind it.

Some of jEdit's features include:

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. See the Features page for a full list.