The Elder Scrolls III: Morrowind

The Elder Scrolls III: Morrowind is a freeform, first-person roleplaying game, which gives the player freedom to travel and act as they wish in the game world. The main plot is linear, but can be progressed through at whatever pace, or even ignored completely. The game doesn't impose any "unrealistic" boundaries, such as not allowing the player to climb over a mountain or swim too far away from shore (except for the Tribunal expansion).

The game features a large and well detailed world as well as a multitude of quests, most of which are unrelated to the main quest and can thus be completed in any order. The downside to the amount of content is that Morrowind suffers from bugs (though most have been fixed by patches/mods) and unexplained crashes to the desktop, especially once the player has played a long time (and consequently modified the world a lot). In addition to this there are many ways to abuse the game mechanics in order to create an unbeatable "god-like" character.