d20 is a generic game system. This means it's not based on a particular film or TV series, or any existing fiction. It simply specifies a set of rules that the GM can use to create their own game.
There's also the d20 Future version of this. It includes rules for things like lasers and starships, instead of just guns and cars.

Pros

Versatility. Do whatever you want. Literaly.

Cons

Requires a highly skilled GM, since the game isn't set in a particular universe, so the GM must explain the setting in great detail and provied an enthralling adventure.

Publisher: Wizards of the Coast

Helpful links:

Adventure so far

Only one game of this has been played.

A band of investigators, psychics and heavies are gathered and sent to investigage a mysterious series of events around Loch Ness.