JEdit has lots of plugins that help you customize the editor to your liking. The plugins are easy to download (once you select a mirror) and take effect right away, without a restart.

Though first released in 2007, it still gets updates quite frequently.

jEdit runs on Linux, OS X and Windows.

Java was the inventor of complete cross-platform compatibility, and apart from the Java plug-in, which isn't used any longer, is very secure and one of the most popular languages and platforms to learn due to its robustness, flexibility and complete object-oriented design. Because jEdit is written in Java, it can run on virtually any platform.

JEdit offers syntax highlighting for several languages.