Here is a summary of my readings on FLTK and Xforms:

     Both FLTK and Xforns originate from the same older /Forms/ library.

     Both are available from debian main.

     FLTK is OO and written in C++. It has Perl, Python, Lua, Ruby and 
Tcl bindings. Xforms is written in C. No binding is listed on the 
project's site.

     Both libraries come with a GUI designer application, FLUID for 
FLTK, and fdesign for Xforms.

     Both are cross-platform: all *nix, MacOS, Windows (maybe not all 
versions of Windows, for Xforms). Both toolkits are available in Debian. 
Both have an opengl interface and FLTK also has a cairo interface.

     These two libraries are lightweight, but their style is fixed and 
cannot follow the customization of the DE. On the other hand, they have 
the same style on all platforms.

     Amusing: Xfce was originally based on Xforms, before it switched to 
Gtk+.

     Didier