RuneScape
RuneScape actually predates MapleStory, as it was released in beta in 2001 as a Graphical Multi-User Dungeon. The gameplay is similar to contemporary MMO games, notably EverQuest and Ultima Online, focusing equally on non-combat and combat skills. However, as MMORPGs became more popular and refined, RuneScape 2 and RuneScape HD, released in 2004 and 2008, respectively, resemble games like MapleStory, with an emphasis on leveling up and questing with groups of other player characters.
Ragnarok Online
Released in 2002 by Gravity Co., Ragnarok Online is also a 2D, sprite-based MMORPG. Notably, it is displayed in an isometric top-down perspective, rather than a side-scrolling game. It also focuses on jobs and classes, and co-operative questing with other player characters. Unlike MapleStory, Ragnarok Online has both F2P servers and subscription servers that can be switched between as any player wishes.
Guild Wars
Guild Wars is a F2P MMORPG that is distinct from MapleStory in a few key areas. The game must be purchased; but, after it's installed, there is no subscription fee. Guild Wars is also fully 3D and thus supports fewer characters per instance. In almost every other way, the two games are very similar, with robust player v. enemy and player v. player combat options, customizable avatars and class-based game play.
Mabinogi
Mabinogi is another MMORPG developed by Nexon that has similar graphics to MapleStory. However, this game is fully 3D and contains a subscription option. Like most of the F2P 3D games, it supports fewer players per instance. In most respects, the focus of this game is similar to Maplestory in character customization, jobs and classes, non-combat skills and social interaction.