nGui provides GUI support for the Neko and haXe platforms. The library is small, but simple to use and provides the standard controls most standard applications require.
If you need something larger, see Blackdogs hxGtk library. You can find this in the haXe website's project section.
ATTENTION!
I really could do with some makefiles for this project for Mac OSX (both architectures) and Linux. Please, if you're good at that sort of thing, lend a hand :-)