In celebration of the 15th anniversary of The Elder Scrolls series, Bethesda Sotfware has made The Elder Scrolls II : Daggerfall available for free download.In Daggerfall, your character is sent by the Emperor to free the ghost of King Lysandus and find out what happened to a letter that the Emperor sent to a Blades spy in the Daggerfall court.
The world you play in is large (if a bit monotone), with hundreds of towns and dungeons to travel to. The usual guilds available for you to join again, with many quests you can complete to rise in ranking in the guild.
You will need an emulator like DosBox to run this game, which is also free to download. Take some time to play the forerunner of Morrowind and Oblivion, I've found the game to be great fun.