A free multi-body vehicle simulator
One point of difference between Slipstream and every other motorcycle racing simulator I'm aware of is that it does not try to control the vehicle for you in any way. You can't just expect to press the left arrow key and magically balance the motorcycle at a 50 degree lean angle. Slipstream just simulates the actual machine piece for piece and allows you to use your mouse to apply pressure to the steering bars and control levers, the rest is up to you. This makes it a lot harder to learn how to control your vehicle but it also makes it a lot more fun in my opinion by bringing out all the interesting effects most motorcycle riders are familiar with. Open the throttle too much when you're leaned over and the bike will drift, drop the front wheel without finesse after a wheelie and you'll get a tank-slapper, hit the brakes too hard and the bike starts weaving as the rear wheel loses traction. I even managed to "perform" a high-side although to be honest I was trying for it.
Another unique point about Slipstream is that the vehicles are modeled piece for piece and are completely configurable. The current motorcycle model has been based on a couple of academic research papers that describe a particular 1000cc motorcycle so the default configuration reflects that. But that doesn't mean that only one motorcycle is available. The graphical interface allows the separate configuration of every parameter so that it's a matter of minutes to, for example, raise the steering head angle a few degrees, move the front wheel mount point further to the front and get a chopper-style motorcycle. Raise the rear spring stiffness to the point of rigidity as well and you get a hard-tail. Or you can leave everything as is and just move the rear wheel mount-point more to the back to get a long-swingarm drag-bike. You can probably make modifications that actually make the bike perform better as well. The possibilities are endless.
Apart from that, the fact that you can make modifications and try them out in real-time should make Slipstream ideal for anyone wanting to study the nature of motorcycle dynamics. It is easy for example to change the steering head rake or offset and see how this affects the handling. For the more academically-inclined it is also very easy to study the motorcycle by recording some parameter and then producing plots. For example it's easy to change to front chain sprocket position and see how this affects motorcycle squatting during acceleration by recording the extension of the rear damper rod during scripted acceleration runs before and after the modification.