1. Minecraft (AnjoCaido Launcher)
2. World of Tanks
3. RuneScape
4. Kerbal Space Program (There is a free demo of the game under the "Downloads" tab of their website)
5. Battlestar Galactica
RotMG for sure. Bad Eggs is also pretty good, even though it isn't exactly PC. It is multiplayer, though. The Old Republic is interesting. Gameplay isn't great, but the cutscenes are awesome.
1: Star Wars: The Old Republic
2: Lord of the Rings Online
3: Dungeons and Dragons Online.
4: Minecraft (free version)
5: Anything on this good old website of Armor Games