What's everyone's favourite games that are entirely free to play? My personal favourite at the moment is probably Realm of the Mad God, which is a massively multiplayer fantasy shoot-em-up with players facing off against monsters in different areas with the ultimate aim of killing the server's god, at which point that server resets. It also has permadeath.
Dwarf Fortress is a really good management game as well, so long as you don't get annoyed to death by your dwarves locking themselves on the wrong side of floodgates as you're filling them with lava and so on. It's got a really complicated industry system, but thankfully you can ignore most of that at the start.
Other games which I've enjoyed included Runescape, but after a while the limitation of the free to play mode there get too frustrating to keep playing, and Minecraft Classic (of course, the problem with Minecraft Classic is that Minecraft itself has pretty much sapped away all the players now, so the free to play version isn't as enjoyable now as back before I bought Alpha.)