http://www.platogo.com/home
http://www.bonuslevel.org/games/
Before you ask these are not typical browser game websites, these are websites devoted to having the users create the levels for the games, in other words the games start out not having much in the way of levels, but they get plenty real quick because the fanbase makes them.
I personally think this could be the future of browser games, having the users create levels for browser games is a great idea because it really jacks up the re-playability of the games. Most browser games get boring after 5 minutes of playing the same collection of levels over and over if you try again and again and then complete the game. Now if you’re stuck on a level you can go to another user-made level and try it instead.
It’s a great experiment, but one I think will bear many fruits and will be responsible for spectacular amounts of wasted time. Try some of the games and you may think the user creating part may just be a great idea.
