Algodoo

Entirely my own creation, though it is of course based on several real-world machines. Is tuned to the 5kg "boulder," and can't throw anything heavier than 7 or so, but it throws the 5kg hard. Hard enough that I had to delete the right killer plane or it would hit it.

It's not incredibly accurate but it rarely misfires. Angle is adjustable from maybe 35º to basically straight up (if you want that for some reason). Move the release mechanism with w/s. It can't go lower than ~35º without a taller (and rule-breaking) release mechanism, but if you scale that it should be able to throw low. It's not tuned for that, though, and the wheel would probably hit the edge and make it not throw very hard.