GTK+, The GIMP ToolKit
 
Cross-platform Graphical User Interface library

Website: http://www.gtk.org
Platforms supported: Win32, Linux
Headers to include: gtk/gtk.bi
Example Usage: yes, in examples/GUI/GTK+/
Header version: 2.24.27, 3.14.10

By default, gtk/gtk.bi will use the GTK+ 2 API.
Define __USE_GTK3__ before including gtk/gtk.bi to use GTK+ 3.

Example

#include once "gtk/gtk.bi"

Dim Shared As GtkWidget Ptr win

Private Sub on_clicked cdecl(ByVal button As GtkButton Ptr, ByVal userdata As gpointer)
    Static As Integer clickcount = 0
    clickcount += 1
    gtk_window_set_title(GTK_WINDOW(win), "clicked " & clickcount & " times")
End Sub

gtk_init(NULL, NULL)

win = gtk_window_new(GTK_WINDOW_TOPLEVEL)
gtk_window_set_title(GTK_WINDOW(win), "A small GTK+ example")
gtk_window_set_default_size(GTK_WINDOW(win), 300, 200)
gtk_container_set_border_width(GTK_CONTAINER(win), 20)

g_signal_connect(G_OBJECT(win), "destroy", G_CALLBACK(@gtk_main_quit), NULL)

Dim As GtkWidget Ptr button = gtk_button_new_with_label("Click me!")
gtk_container_add(GTK_CONTAINER(win), button)

g_signal_connect(G_OBJECT(button), "clicked", G_CALLBACK(@on_clicked), NULL)

gtk_widget_show_all(win)

gtk_main()