I used to be into browser-based games pretty good. In my school, RWK had some massive popularity to the point where I know at least two people who had forked over upwards of $100 for premium stuff. I haven't played it in probably three years, but it's pretty neat.

I also remember one called Betholia that was pretty fun, but it didn't last long. I think I searched for it recently but couldn't find much except something about it coming back soon. I kinda liked that one.