jEdit rocks because:
It's Java, so it works on Mac, Win, Linux|Unix|BSD|Solaris, etc.
It can be as simple or as feature-rich as you need, with vast syntax highlighting/coloring, possible code completion, source code control integration, etc. The plugins let you create your editor.
It's not huge - the download is ~2MB, and installed it's ~4-12MB depending on options.
It doesn't put