http://www.openwatcom.org/
The Watcom C/C++ compiler (now OpenWatcom C/C++ compiler) is now open source and free. This compiler, which generates code for Win32, Windows 3.1 (Win16), OS/2, Netware NLM, MSDOS (16 bit and 32 bit protected mode), etc, was a highly coveted compiler some years back (until Sybase terminated it). The compiler also includes the rather well-known STLport (a C++ Standard Template Library implementation).

http://www.borland.com/products/downloads/...d_cbuilder.html
Borland has released their C++ Compiler (version 5.5 at the date of this writing) FREE! You get the command line compiler, incremental linker, resource compiler and binder, C++ Win32 preprocessor, import library utility, librarian, obj/lib/exe dump utility, etc. Included is also the C/C++ runtime library and ANSI/ISO Standard Template Library (STL). You can also get the Borland Turbo Debugger to debug your programs separately (see our Free Debuggers, Source Code Checkers, Bug Trackers, Memory Leak Detection page). You can also get older versions of Turbo C (such as Turbo C 1.5 and Turbo C 2.0) from Borland's Community Museum.

http://msdn.microsoft.com/netframework/dow...es/default.aspx
Unbelievably, the Microsoft Visual C/C++ command line compiler, along with C#, VB.NET and JScript.NET, appears to be available from Microsoft for download for free. (Note that if you use the Opera web browser to go to this URL, set Opera to pretend that it is Mozilla (just hit F12 and select Mozilla 5.0), otherwise Microsoft's web server will redirect you to a dummy page. Their browser detection script, which serves different pages to different browsers, is broken.) You will also need to download the Microsoft Platform SDK which contains the Windows headers and libraries for the compilers. The command line compiler is the same one that comes with the commercial Visual C++ Standard, which means that it does not have an optimizer (or at least, not the optimizer that ships with the Professional version that comes with MSDN).

http://root.cern.ch/root/Cint.html
This is a C/C++ interpreter that comes with source code. It can be compiled and used on a large variety of Unices, including HP-UX, Solaris, SunOS, AIX, Alpha/OSF, FreeBSD and Linux, as well as Windows NT. You can even issue system calls from the interpreter (such as Posix calls on Unix or Windows API calls from Windows). It accepts most of K&R and ANSI C and is close to the standard C++ (at the time of this writing). As with all interpreters, it may be useful in debugging programs or learning the language.

http://www.z88dk.org/
z88dk is a z80 C cross compiler based on the Small C compiler, supporting many features of ANSI C. It comes with an assembler and linker as well as a standard C library. Supported host systems include Amiga, BeOS, HP-UX 9, Linux, BSD systems, MacOS X, Solaris, Win32, Win16 and MSDOS. The compiler generates code for the following target systems: Cambridge Computers z88, Sinclair ZX Spectrum, Sinclair ZX81, CP/M based machines, Amstrad NC100, VZ200/300, Sharp MZ series, TI calculators (TI82, TI83, TI83+, TI85, TI86), ABC80, Jupiter Ace, Xircom REX 6000, Sam Coupe, MSX1,Spectravideo, Mattel Aquarius, Peters Sprinter, and C128 (in z80 mode).