Ada Interface to MindStorms
Price: Free
Language(s): Ada
Homepage: http://www.faginfamily.net/barry/Papers/AdaLetters.htm

brickOS
Price: Free
Language(s): C/C++
Homepage: http://brickos.sourceforge.net/
Comment: A library for GCC, you need to build GCC yourself of use QuiteC's binaries

GCC
Price: Free
Language(s): C/C++, Objective C, Fortran, Java, Ada among others
Homepage: http://gcc.gnu.org/

GNU Toolchain for h8300
Price: Free
Language(s): C/C++, ASM
Homepage: http://h8300-hms.sourceforge.net/

Lego.NET
Price: Free
Language(s): Anything that can compile to .NET, works best with C#
Homepage: http://www.dcl.hpi.uni-potsdam.de/research/lego.NET/
Comment: Doesn't come with a compiler, converts bytecode to machine code

Legolog
Price: Free
Language(s): Prolog
Homepage: http://www.cs.toronto.edu/cogrobo/Legolog/
Comment: Uses NQC to interpret commands and sends them from the PC to the RCX via IR

leJOS
Price: Free
Language(s): Java
Homepage: http://lejos.sourceforge.net/

librcx
Price: Free
Language(s): C/C++
Homepage: http://graphics.stanford.edu/~kekoa/rcx/tools.html#Librcx
Comment: A library for GCC

Logitech SDK
Price: ?
Language(s): Visual Basic, C/C++
Homepage: http://www.logitech.com/index.cfm?PAGE=partners/select&CRID=932&countryid=18&languageid=1
Comment: Can be combined with an RCX control library such as spirit.ocx from the MindStorms SDK to make use of the Lego Cam

NQC
Price: Free
Language(s): NQC, a C-like language
Homepage: http://bricxcc.sourceforge.net/nqc/
Comment: This is the most widely used unofficial language

Official MindStorms SDK
Price: Free
Language(s): Visual Basic, Visual C++, MindScript, LASM
Homepage: http://mindstorms.lego.com/sdk2point5/default.asp
Comment: You don't need VB to use the VB features as MS Office comes with a cut down version of VB for making macros

OnScreen
Price: Free
Language(s): A custom language which can be programmed directly on the RCX
Homepage: http://alumni.cse.ucsc.edu/~davevw/onscreen/index.html

pbForth
Price: Free
Language(s): Forth
Homepage: http://www.hempeldesigngroup.com/lego/pbForth/

PbrickDev
Price: Free
Language(s): Custom flowchart-based language
Homepage: http://www.geocities.com/pbrickdev/mainpage.html
Comment: Has more fuctionality than the RIS language, such as datalogs, subroutines and multiple threads

PRO-BOT
Price: Free
Language(s): A kind of Visual Basic/spirit.ocx-based language
Homepage: http://mapageweb.umontreal.ca/cousined/lego/4-RCX/PRO-BOT/index.html
Comment: Designed for robots which are in contact with the workstation at all times

QuiteC
Price: Free
Language(s): C
Homepage: http://www.elenafrancesco.org/info/lego/qc/index.html
Comment: A library for use with GCC and comes with GCC for Windows

RCX Code
Price: Cannot be bought on its own, included in RIS
Language(s): RCX Code, a custom flowchart-based language
Homepage: http://mindstorms.lego.com/eng/products/ris/rissoft.asp

ROBOLAB
Price: ?
Language(s): A flowchart language based on LabVIEW
Homepage: http://www.lego.com/eng/education/mindstorms/home.asp?pagename=robolab
Comment: This is the programming environment offered to schools who use MindStorms, supports the Lego Cam

SqLego
Price: Free
Language(s): Squeak
Homepage: http://minnow.cc.gatech.edu/squeak/2412

TclRCX
Price: Free
Language(s): Tcl
Homepage: http://www.demailly.com/tcl/rcx/

Terrapin Logo
Price: $99.95 (US)
Language(s): LOGO
Homepage: http://www.terrapinlogo.com/show.cgi?id=1060225103441218&file=software/logo.html

TinyVM
Price: Free
Language(s): Java
Homepage: http://tinyvm.sourceforge.net/

The Transterpreter
Price: Free
Language(s): occam
Homepage: http://transterpreter.org/

Vision Command
Price: Cannot be bought on its own, included in Vision Command expansion pack
Language(s): RCX Code
Homepage: http://mindstorms.lego.com/eng/products/vc/index.asp
Comment: The official programming language for use with the Lego Cam

XS
Price: Free
Language(s): Lisp
Homepage: http://www.yuasa.kuis.kyoto-u.ac.jp/~yuasa/xs/