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 (free, terribly buggy, doesn't work on 64-bit, beloved by all)
2. Scratch (free, multiplatform, designed as an educational tool)
3. Construct Classic (free, open-source, Windows, not really updated)
4. Stencyl (free lite version, multiplatform, produces Flash games)
5. Game Maker (free lite version, Windows)
6. Ren'Py (free / open-source, crossplatform everything, great for visual novels, questionable for anything else)
7. Adventure Game Studio (free, games vaguely crossplatform, editor Windows-only)
8. Multimedia Fusion / The Games Factory (François Lionet sez: Get $20 off everything with coupon code GTFGR4U. Good 'til June!)
9. Construct (HTML5 output, free lite version)
10. Processing (free, multiplatform, simple Java-ish programming environment)
11. Unity (free, multiplatform, 3D, requires programming)
12. ---- TOOLS BEYOND THIS POINT ARE WEIRD ----
13. BYOND (free, networked)
14. VERGE (free, RPG-oriented, crossplatform)
15. MegaZeux (free, textish, platforms?)
16. ZZT (DOS, even more textish)
17. Scrolling Game Development Kit (free, Windows)
18. Scrolling Game Development Kit 2 (free, Windows)
19. Ray Game Designer 2 (free, 3D)
20. ZGameEditor (free, 3D, crossplatform, procedural)
21. Novashell (free, crossplatform)
22. DS Game Maker (Nintendo DS)
23. AMOS / STOS (Amiga / Atari ST)
24. Garry Kitchen's GameMaker (C64)

Anything else worth looking at?