The otherlegOS homepage. This "official" page is used mainly for CVS and file download, since sourceforge provides 100M of free storage and unlimited downloads. It will (eventually) be the "official" site for all things legOS, including development and documentation.
LUGNET- the Internet's home of all things Lego. See particularly the robotics mailing list and the LegOS mailing list.
Lego Mindstorms Internals: This page has tons of links, focusing mainly on OS-type programs and communications protocols.
MIT Constructopedia: A work in progress at the MIT Media Lab.
O'Reilly Link Page: Jonathan Knudsen, writing for O'Reilly Publishing, has created a Mindstorms book. This page has all the links from the book.
Mario Ferrari's Lego page. Some of these are legOS powered, and there are lots of neats things legOS and non-legOS.
Bert van Dam's page: heavy AI stuff, including a neural net and lots of links.
Simen Skrogsrud's page: He built a scanner out of Lego. What more do you want?
Alex's page: some good designs based on the "differtrans," a transmission which uses the differential unit included with the Mindstorms kit. A good place to start if you are very concerned about traveling in a straight line.
Machina Speculatrix: A page discussing a very early "learning" robot, with an attempt to replicate it in the RCX. Very interesting. Part of the Gasperi web complex :)
Clint Rutkas' page: A page with some good designs, including a large, functional claw/arm combination. Worth checking out.
Rich Thompson's page: you know you are serious when you get your own URL :)
Joe's Mindstorm's Gallery: unfortunately, this page is in Japanese, but if you look, you'll find some gems- including a Lego-based Turing Machine.
The Mill Page: Picture a Lego mill cutting designs into burnt toast. An interesting experiment in Lego control.
Dave's Page: if nothing else, check out the Johnny 5 replica.
Jin Sato's page: Another Japanese page. Excellent pictures of an arm and a robot which rotates in place.
Fred Martin's MIT page: check out in particular the Robot Builder's Guide. The same author (who was a co-founder of the MIT class) has also written "The Art of Lego Design," available in PDF and compressed PostScript. This is an expanded and better illustrated version of chapter 7 of the Robot Builder's Guide, which I mentioned in Section 2.4 of this document.
RCX Arena Combat: some ideas for interactive Lego competitions.
A Robotic Vacuum Cleaner: Should be very easy to do with LegOS, but Eureka wants to charge $1,000 dollars a piece for it.
This list has now evolved into the Lego + Linux mini-HOWTO, available separately from the Linux Documentation Project. I urge Linux users to look there first, since it is (most likely) more up to date and definitely better organized and more informative. Windows users may also want to take a look, since most of the projects described there have Windows ports.
NQC: this is a C-style language that uses the standard Lego firmware. This makes it easier for beginners, but much less powerful for experienced coders, since there can be only 32 variables, and there are limitations on threading and concurrency.
pbFORTH FORTH is a reasonably common scripting language, for which Ralph Hempel has written an interpreter which fits on the RCX. This means no compiler on your machine. Furthermore, the latest version allows cooperative multi-tasking and interactive debugging. You can also link sophisticated scripts in order to allow for a basic form of libraries. This has many of the strengths of LegOS, since it has unlimited variables and threading. Only substantial drawback, as far as I can tell, is learning FORTH :) Ralph tells me there are good tutorials linked to from his site, but I have not explored them.
Smalltalk: Don't know much about this at all. Let me know!
TinyVM: An actual Java VM for the RCX. That's right: write Java, compile on the PC (or a Sun box) and download to the RCX to run. Not yet as sophisticated as legOS, but catching up fast.
Lego::RCX.pm: If Java or C just aren't your style, TMTOWTDI. That's right, write in Perl to control the RCX. Unlike TinyVM or legOS, this is just a way to send commands to the robot from your computer. However, that can still be very useful; in fact, some would argue more useful. Whatever :)
Jini: Program the RCX in Jini, Sun's Java-based distributed systems architechture. There was a similar document for J++ 6.0 at Aviv Eyal's site., but the entire site is 403: Forbidden at this time.