Every MMORPG that makes it big today is stamped as being a World of Warcraft clone. The thing is, WoW wasn't exactly an original idea itself. Before WoW, there were a handful of hugely successful MMORPGs: Ultima Online, Asheron's Call, Dark Age of Camelot, and EverQuest. EverQuest was really the second commercially successful MMORPG, behind Ultima Online from 1997, but there's no doubt that it brought massive success to the genre as the first MMORPG on a 3D game engine.
Released in March of 1999, EverQuest has branched out into several titles like Lords of EverQuest, Champions of Narroth, EverQuest II, and the (unfortunately) terminated title that we all thought would retake the MMORPG scene by storm, EverQuest Next.
Before there was WoW, there was EverQuest. Without it, who knows if any of the MMORPGs thereafter would have seen such impressive numbers.