Hey, let's have a thread listing all of the rapid game development tools we know about! I'll keep the list updated.

1. Klik N' Play (freeish, terribly buggy, beloved by all)
2. Construct (free)
3. Scratch (free)
4. BYOND (free, networked)
5. Novashell (free, crossplatform)
6. Scrolling Game Development Kit (free, Windows)
7. Scrolling Game Development Kit 2 (free, Windows)
8. Adventure Game Studio (free, games vaguely crossplatform, editor Windows-only)
9. Game Maker (free lite version, Windows)
10. Ray Game Designer 2 (free, 3D)
11. ZGameEditor (free, 3D, crossplatform, procedural)
12. Processing (free, Java)
13. MegaZeux (free, textish, platforms?)
14. ZZT (DOS, even more textish)
15. VERGE (free, RPG-oriented, crossplatform)
16. Multimedia Fusion / The Games Factory
17. Unity (MacOS editor, crossplatform deployment)
18. DS Game Maker (Nintendo DS)
19. AMOS / STOS (Amiga / Atari ST)
20. Garry Kitchen's GameMaker (C64)

Anything else worth looking at?