http://geany.org

Geany is a GPL TextEditor / lightweight IDE for UNIX/Linux using GTK. A Windows installer is also available that bundles the required GTK libraries.

It has the only implementation I've seen of something very useful, a checkbox to invert the colors to give a quick white-text-on-black editor.

Here's it's feature list from Wikipedia:

* Auto-completion
* Multiple document support
* Project support
* Syntax highlighting
* Code folding (partially)
* Symbol lists
* Code navigation
* Embedded terminal emulator
* Customizable build system to check, format, compile and execute code
* Send selection to external tools
* Extensible via plugins

It has plugins for VersionControl integration (*nix only), scripting with LuaLanguage, online documentation browsing and window splitting.