A port of the Unix editor, ships with 297 different syntax colouring files (including XML). Use GUI or as ANSI task with Nettle.
Vim is covered by the Vim License, compatible with the GPL.
Vim is a cross-platform text editor which ships with a very large number of syntax highlighting files (297 with this version, more with 6 onwards). This does necessitate a filing system which can cope with >77 objects per directory, e.g. raFS or RISC OS 4's E format.
Vim is customizable and you can use your config files from other platforms too (as well as those found around the WWW). It also makes use of virtual memory so the size of files you can edit is only really limited by your harddisc space.
Vim has been tested as the GUI version as well as an ANSI task with Nettle, and should also work in full screen mode too. Version 6 will follow when the GUI code is working again.
The direct download link has been removed as Vim comes in two main archives and you should check whether you need to update either. The syntax files should now remain untounched unless bugs are found in them.