FreeBASIC
FreeBASIC is a free/open source (GPL), 32-bit BASIC compiler for Microsoft Windows (32-bit), protected-mode DOS (DOS extender), the Xbox and Linux (x86), and is being ported to other platforms.
It makes use of the GNU binutils programming tools as backends and can produce console, QuickBasic-compatible graphical and GUI executables, besides dynamic and static libraries. A GCC frontend is currently being developed, which will enable compilation for many more operating systems than currently supported.
Currently, software libraries like GTK+, GSL, SDL, Allegro, Lua and OpenGL can be used directly (no wrappers, only function and structure prototypes), and more are being added on every new release. A complete set of the Windows API headers is also included.
The compiler, and its nearly 90,000 lines of code, was, and still is, compiled by itself (FreeBASIC is a self-hosting compiler). The first version of FreeBASIC was written in Visual Basic for DOS for that purpose.