The Warcraft 3 scenario Defense of the Ancients is probably the most popular scenario out there, Well, The same guys that made DOTA have made a whole game out of it. It's got quite a following and if you like DOTA, you'll probably like LOL. It still pretty much looks and plays like the Warcraft 3 map.
Edit: I took out the beta stuff from this post to avoid confusion. The game is out now and free to play. Thanks go to Cynthetiq and Zeraph for gettin the title changed.
League of Legends - Index