I think Will Wright's Spore is going to knock this space wide open. It's not web-based, but it is multi-platform and web-enabled. Spore focuses on procedural user-generated content and sharing it across devices as diverse as phones, Nintendo DSes, Wii, PC, and other platforms. It's not completely a game creation toolkit because it doesn't offer the in-depth control that programming offers. But it's related in that Wright's philosophy is to provide an open-ended 'toy' which enables players to create their own game. As a gamer and hacker myself I look forward to seeing this trend develop further.