FLTK or wxWidgets. wxWidgets on Linux is actually wxGTK though, but it's pretty painless. Or you could use Qt, which doesn't really look native on other platfor

FLTK or wxWidgets. wxWidgets on Linux is actually wxGTK though, but it's pretty painless. Or you could use Qt, which doesn't really look native on other platforms. And another one: Xlib hahaha lol