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. Twine (free / open-source, creates web-based text games with a nice no-programming GUI interface)
3. Scratch (free, multiplatform, designed as an educational tool)
4. Construct Classic (free, open-source, Windows, not really updated)
5. Stencyl (free lite version, multiplatform, produces Flash games)
6. Game Maker (free lite version, Windows)
7. Ren'Py (free / open-source, crossplatform everything, great for visual novels, questionable for anything else)
8. Inform 7 (free, creates multiplatform interactive fiction)
9. Knytt Stories (free, Windows, flexible platforming level editor)
10. Adventure Game Studio (free, games vaguely crossplatform, editor Windows-only)
11. Multimedia Fusion / The Games Factory
12. Construct (HTML5 output, free lite version)
13. Processing (free, multiplatform, simple Java-ish programming environment)
14. Unity (free, multiplatform, 3D, requires programming)
15. ---- TOOLS BEYOND THIS POINT ARE WEIRD ----
16. BYOND (free, networked)
17. VERGE (free, RPG-oriented, crossplatform)
18. MegaZeux (free, textish, platforms?)
19. ZZT (DOS, even more textish)
20. Scrolling Game Development Kit (free, Windows)
21. Scrolling Game Development Kit 2 (free, Windows)
22. Ray Game Designer 2 (free, 3D)
23. ZGameEditor (free, 3D, crossplatform, procedural)
24. Novashell (free, crossplatform)
25. DS Game Maker (Nintendo DS)
26. AMOS / STOS (Amiga / Atari ST)
27. Garry Kitchen's GameMaker (C64)

Anything else worth looking at?