I have been playing with the jQuery animate functionality a bit today and it occurred to me that jQuery would likely be the perfect library to build a browser based game on top of..
I did a little browsing about the web, and sure enough.. someone thought of it before me.
The games I would want to create would be less animated than this one.. but I could totally see a Masters of Orion remake as a browser based game using the jQuery library.. Or any number of other turn based games.. You could even make them multiplayer fairly easily..
More over you wouldn't have to sell any kind of client.. everyone with a modern computer has a web browser.. You trade having to create a game that works with hundreds of different graphics drivers and computer configurations for having to deal with 5 major browsers that all do things slightly differently, but big deal.. jQuery does most of that nicely, so really you're only stuck fixing CSS issues..
As always the trick is in coming up with a game that is compelling enough that others want to play it. I mentioned Masters of Orion above because I think that's likely one of the best games of all time and as such a perfect candidate for remake in this format.
Granted you're not going to make the next 3D masterpiece as a browser based game.. but I say good.. Games these days are moving towards looking good with zero game play.. Give me a game that is fun to play any day over the latest wiz-bang graphics.. I think the browser game market is a good place to try this..