Here are the corresponding threads to a couple of the more popular levels.
Please post your solutions to these levels in their corresponding threads, so we can all marvel at your fantastic contraption.
11. Mission to Mars - viewtopic.php?f=3&t=23
13. Big Ball - viewtopic.php?f=3&t=10
14. Four Balls - viewtopic.php?f=3&t=5
15. Down Under - viewtopic.php?f=3&t=22
16. Awash - viewtopic.php?f=3&t=19
17. Handling - viewtopic.php?f=3&t=34
18. Tube - viewtopic.php?f=3&t=8
19. Back and Forth - viewtopic.php?f=3&t=21
20. Unpossible - viewtopic.php?f=3&t=31
(Thanks for the numbering, Heloed)
(If there are other level specific threads, post them here and I will edit my original response
