Battle for Wesnoth
Free turn-based fantasy strategy. Units have special ability, able to level up (some kind of upgrade), many unique factions (humans, elves, orcs, dwarves, undeads etc.), single player campaigns and free internet multiplayer mode and mods!
Download (300 MB):
Heroes of Might and Magic Online
Sadly only available in Asia, but it is really a good one! Rich gameplay, nice graphics and it is an official HoMM game!
Might and Magic: Heroes Kingdoms
Official game from Ubisoft. Some kind of simplified Heroes of Might and Magic 5, in browser. No registration required!
King's Bounty (original)
Forerunner of Heroes of Might and Magic series. Abandonware, you will need a DosBox too!
Download (650 KB):
King's Bounty: Legions
Are you bored because of poor quality games on Facebook? Check this out! King's Bounty: Legions is deserve its name, really nice strategy gem. Turn-based fights, nice world map, quests, artifacts and everything else!
Video (only video is russian, game is english):
Castle of Heroes
Free Heroes-like MMO, no download required!