League of Legends is a DotA spinoff game made by the same people. Free to download and play. You unlock champions with points earned in game, but you can buy them with real money too, but not necessary.
I like it because you can play one game that lasts maybe 45 minutes and then leave it there. Not a massive time-sink like other games (WoW) and ideal for me as I don't want to spend excessive amounts of time in front of the PC. Perfect for just unwinding a bit at the end of the day and over weekends. Also, no peer pressure.
Checked out Dream of Mirror Online a while ago as well, but was not impressed by it. The best thing about it is the anime-style characters, but absolutely no gameplay value for me.
LOL is not a MMO it is a MOBA (massive online battle arena). Heroes of Newerth is better though, and yes Icefrog worked on it! Pendragon worked on LOL