I recently found Quake Live. Okay, so I’m a bit behind on this one. The open beta launched February 24, 2009 on www.quakelive.com. As stated in this article, Quake Zero not actually in web-browser, it’s actually not running in the browser, but rather in its own window. To play you have to install a plugin. Currently, only Internet Explorer 7+, Mozilla Firefox 2+, and Safari 3+ web browers running in either Windows XP, Windows Vista, Windows 7, Mac OS X 10.4+ (Safari only), and Linux are supported. -Wikipedia

In August, Microsoft unofficially (through a job post description) announced they are going to have a more widespread access to their gaming network:

The LIVE Engagement Team is looking for a LIVE Community Director to manage its LIVE community strategy and execution across a range of properties, from Xbox LIVE to Windows Mobile. This senior position will play a vital role in the community space as the LIVE Engagement team builds and program’s Microsoft’s next-generation, LIVE-enabled casual and social gaming platform across the Web, the console, mobile and beyond.

That is not to say they are going to have the actual games on there, but it’s sure a step in that direction.

At this point, a majority of games have an online multiplayer mode, so the transition would really just be a small step. Distribution would be free and much easier. And then there is the viral effect of social networks.

Update: Another interesting web-based game is Monopoly City Streets available on http://www.monopolycitystreets.com/ It makes use of Google Maps.