Another link, Amigo has a bunch of gtk 1.2 apps ..... http://distro.ibiblio.org/pub/linux/distributions/amigolinux/download/Applications/ .
There's mention in an old Cinepaint thread ( http://osdir.com/ml/video.cinepaint.devel/2004-04/msg00009.html ) about FL-GTK, a compatibility layer for GTK 1 applications to build using FLTK. FLTK's been through some changes, so a library like this would probably need an update. However, FLTK 1.3 now has some internationalization (Unicode and UTF-8) support which is a plus. I'm currently working with a patched version of FLTK 1.3 that offers themes by setting the FLTK_SCHEME environment variable. So, there is some theme changing capability in version 1.3 too. It would be really nice if we could port some useful GTK 1.2 applications over to FLTK 1.3. There are also quite a few applications that already work with FLTK including a desktop (Equinox Desktop Environment). Other applications available for FLTK include graphics editors and viewers, a beta version of Cinepaint Glasgow, flwriter (word processor), two lightweight browsers (dillo and dplus), a xine front end (multimedia player), daily journal program, todo and disk usage utilities, minimal file manager, alsamixergui, games and a nice front end to synclient for touchpad users.
If anyone would be interested in something like this, I'd be very willing to help attempt to port some other applications over to FLTK 1.3.