DOS/Windows and Macintosh users are often not aware of the vast world of free (meaning that you are free to use it, copy it, and give copies of it away) and open-source (meaning that the original source code is provided) software available, largely thanks to the efforts of volunteer programmers from the Unix world, where a culture of sharing software has never been extinguished. The surprising thing to those first discovering the world of free/open source software is not just that there is so much of it, but how good it is. This page contains links to Internet sites where you can find documentation and code for programming-language processors, interpreters, compilers, assemblers, etc., for many programming languages.
I have arranged the information by platform. Proceed to the section for the platform you are interested in, DOS/Windows, Unix/Linux, or Macintosh, and follow the links to the sites for the various language processors.
Bywater BASIC was written by a grandmother who didn't understand why Digital Research didn't sue Microsoft for copying CP/M in the form of MS-DOS. She died a tragic death, but you can honor her by using her BASIC interpreter. (The story is included in the docs.) I have created binary versions for MS-DOS and Windows 3.x and for Windows 9x, NT, and ilk, that you can just download and run. Here's a document that explains some of the peculiarities of BWBASIC, and here's the official documentation.
Dev C++ is a full-featured integrated development environment which is able to create Windows or console-based C++ programs. You can get Dev C++ here, from SRU, too.
DJGPP is a port of the Gnu C/C++ compiler system, the dominant compiler system in the Unix world, to DOS and Windows. It has an integrated development environment and the necessary libraries to create fullblown Windows applications.
LCC-Win32 is a "free" compiler system and IDE for Windows. It also contains a Fortran compiler. There is a charge if you want the source code of the compiler system or the example applications.
Free Pascal is compatible with Turbo Pascal 7.0, with several Delphi extensions.
Python is a cool language, the first language I know of to be pseudocode-friendly, and certainly the only programming language to be named after a comedy troupe. It may be the best programming language for beginning programmers. (Yes, Javascript is nice for beginners, too.) It serves as a nice scripting language, and can even be used to write cross-platform GUI applications, using any of several GUI libraries, such as wxWindows and TkInter. Windows users should get the file python-2.1.1.exe.
If you are running Mac OS/X, you are actually running FreeBSD Unix with a Mac user interface. I don't know this for sure, but OS/X probably already has all of the Unix free language processors.
Macintosh Programmers' Workshop is a complete development environment for C and C++ for Mac OS 7, 8 and 9, and it generates machine code for M68k and PowerPC processors.
An old Pascal compiler for Macs with M68k processors is still available from Apple, but it is not supported. (What do you want for free?)
Python is a cool language, the first language I know of to be pseudocode-friendly, and certainly the only programming language to be named after a comedy troupe. It may be the best programming language for beginning programmers. (Yes, Javascript is nice for beginners, too.) It serves as a nice scripting language, and can even be used to write cross-platform GUI applications, using any of several GUI libraries, such as wxPython and TkInter. Mac users should go to Jack's MacPython page for downloads.
Most common programming languages are included with standard Linux distributions; just be sure to install all of the "development" packages. However, I will add links to specific language processors as soon as time allows.
Bywater BASIC was written by a grandmother who didn't understand why Digital Research didn't sue Microsoft for copying CP/M in the form of MS-DOS. She died a tragic death, but you can honor her by using her BASIC interpreter. (The story is included in the docs.) I have created a binary version for Linux, that you can just download and run.
INTERCAL is the only language I know of that has a COME FROM statement. And don't expect there to be a large market for your skills as an INTERCAL programmer. From the INTERCAL Web site:
So, you think you've seen it all, eh?
OK. You've coded in C. You've hacked in LISP. Fortran and BASIC hold no terrors for you. You write Emacs modes for fun. You eat assemblers for breakfast. You're fluent in half a dozen languages nobody but a handful of uebergeeks have ever heard of. You grok TECO. Possibly you even know
The INTERCAL page also has links to several other "interesting" programming languages: Befunge, BrainF***, Malebolge, and Unlambda.