Stendhal features a huge diverse world ()with cities, forests, mines, mountains and even tropical islands and a lovely 2D graphic. While you explore this world and dungeons, you will gain experience. Several hundred NPCs will support you on your journey by offering quests or items.
You can play Stendhal 1.00 on Windows, Mac and Linux. With Java installed, playing is as simple as clicking "Play Now" on the website. Stendhal is completely open source (client, server, everything) and completely free.