FreeBASIC - as the name suggests - is a completely free, open-source, 32-bit BASIC compiler, with the syntax the most compatible possible with MS-QuickBASIC, that adds new features such as pointers, unsigned data types, inline-assembly and many others.

Features :

A large number of variable types available
Arrays
BASIC Compatibility
Built-in gfx library
Clean Syntax
Completely *FREE*
Create OBJ's, LIB's, DLL's, and console or GUI EXE's
Debugging support
Enumerations (ENUM's)
Escape characters inside literal strings
Function overloading
Inline Assembly
Most of the known C libraries can be used directly, without wrappers
Multi-platform
Multi-threading
Name spaces
Optimized code generation
Optional function arguments
Pointers
Preprocessor
Typedefs
Unicode support
Unlimited number of symbols
User-defined Types (UDT's)
Variable initializers (including arrays and UDT's)