FreeBASIC is a free/open source (the compiler is GPL and the runtime library is LGPL with static linking exception), 32-bit & 64-bit BASIC compiler for Microsoft Windows and LInux and a 32-bit compiler for protected-mode extended DOS.
FreeBASIC is a high-level programming language supporting procedural, object-oriented and meta-programming paradigms, with a syntax compatible to Microsoft QuickBASIC. The FreeBASIC project originally began as an attempt to create a code-compatible, free alternative to Microsoft QuickBASIC.
The compiler, fbc, currently supports building for i386-based architectures on the protected-mode extended DOS, Linux, Windows and Xbox platforms. The project also contains thin bindings (header files) to some popular 3rd party libraries such as the C runtime library, Allegro, SDL, OpenGL, GTK+, the Windows API and many others, as well as example programs for many of these libraries.
On Wikipedia: http://en.wikipedia.org/wiki/FreeBASIC