Well I don't know too many on the scale of Runescape but there's a few small and simple ones I'd recommend that I've played recently:
Desktop Tower Defense: I've spent hours on this before. Build a little fortress and try to eliminate the little guys moving across the desk.
Cursor*10: This one was a real mindf__k. You're basically playing with yourself. You travel through the stages until a certain point, when you'll start again and have to work in collaboration with your last play through that will help you progress, if you chose wisely and time it well. [one of those see-it-to-understand-it games]
The World's Hardest Game: Move from A to B without dying. You might end up visiting frustration in between.
The actual hardest game, imo, was hosted
here but the link's dead now for some reason. It was an ASCII adventure. Can't find another link just yet.
Nes classics emulated through your browser: Last one I can think of.
GeneralForum Arcade deserves a mention too! Though you'll need a fair few more posts before you unlock it.
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I believe
Habbo Hotel can be played through a browser, though it's aimed at younger people. As I said, though, these are a long way off Runescape type games.
Waterproof
Dance!
