OK, basically this is a thread for Free to Play games that you recommend. Note that this means that the MAJORITY must be available for free. Wizards 101, a game I love, for example has only the first city and two quest areas open for free. You can either get a subscription to unlock everything for as long as your subscription lasts (don't) or just buy enough cash-shop currency to permanently unlock areas for all characters on your account forever (yes).
First up is Dungeon Fighter Online, a multi-player side-scroller beat-em-up mixed with leveling and customizable equipment. It's a good way to blow off steam and the combat is genuinely fun. The classes honestly feel different, and the equipment choices really do feel like a choice. Do I want to do more physical damage, or improve my magical damage?
Next up is
Dungeons and Dragons Online. It's essentially what it says on the box, the classes feel true to form, it has an excellent story, the quest chat makes sense, and the different types and builds of characters really feel different. A great-sword wielding Warforged Fighter plays differently than an axe and shield wielding Dwarven Fighter.
Alteil is a F2P online card game. It's a good way to kill time and is simple enough to learn quickly. It's not a very deep game, and a part of it is luck of the draw, but it is a fun way to kill time.