If it's flexibility you need as a programmer then try Editra, which features support for over 60 different programming languages.
Editra is not only free but can be just about anything you want it to be, courtesy of plug-ins. Editra supports syntax highlighting and the focus is on making coding as easy as possible no matter what language you're working in. At the moment, Editra can be prone to instability because it's still in Alpha stage but there's a thriving community of users on the developer forum steadily improving it.
Editra is based on the same principle as Firefox in that it can be extended with plug-ins. The developer warns, however, that it's important to choose the plug-ins that match the version of Python installed on your system. If you are unsure of which version, it is better to use the built-in plug-in downloader in Editra and let it choose for you. This is a far easier way too, because it allows you to drag and drop plug-ins in place.
Editra is still very much a work in progress but it's wide language support and extendability make it a very interesting option for programmers working across languages and platforms.
