We already know about Mega-Fail and that new company, so I won't list them on this list - this is more about dissimilar construction sets and kits, as well as hobby equipment.
I'll also list the compatibility percentages and difficulty. Systems will be listed in order of popularity.
- K'nex (80%) (Easy): As well as K'nex Blox, K'nex shaft elements are compatible with the LEGO Technic line to a major extent.
- Erector (40%) (Advanced): The spacing on Erector metal components allows for bolts to be run through every fourth hole on a Technic piece.
- Any 1-Scale, G-scale, or O-Scale Model Trains (10%) (Advanced): Not really compatible, but with a lot of patience, skill and attention to detail you can build believable structures. Indoor use only!
- Rokenbok RC Construction System (50%-US Only) (Easy): European builders, you're out of luck. This has compatibility with a different building system that is NOT compatible with LEGO. In the US, however, Rokenbok floor plates have a LEGO compatible design.
- VEX Robotics Design Set (50%) (Advanced): As well as the above mentioned compatibility for Erector, VEX's square shafts work well as driveshafts for LEGO equipment. Probably the easiest way to add advanced robotics functions or animation to a LEGO display, so long as you know how to
change flowersprogram in RobotC or Lab-View.
- Pitsco/LEGO Education Tetrix Robotics System (20%) (Intermediate): It's not exactly a non-LEGO system - It's made by TLG subsidiary Pitsco and was designed to be controlled with a LEGO Mindstorms NXT unit. However, for the purposes of this list it counts as non-LEGO. The system includes mounting brackets for the pin-only Technic components. Figuring out how to mount the components is easy. Figuring out where to go from there is, er... not so easy.
- Will update list
Systems that don't work:
- Lynxmotion Servo Erector Set: Although it's similar to the Tetrix system, there are no mounting brackets and the components are much smaller. Trust me - I tried.
- Will update list