Not really going to make a list for you, but a simple example of open world environments is a Perfect World this fits the bill as its a good example of having a large open area to play in.
Most games today have maps inside maps! I think mapping is done purely because they don't want to have a server system in place that is able to stretch the limits. But what they didn't realise was that a more freely open world is less load on the server than a map based one wit a map based system your purely enclosed in one space, in an open world with new technology or advancement in gaming allows you to be in an area which does not affect any other part of the world other players are in.
Games like Eve are pure mapping, originally when CCP started it was low budget game, un/fortunately for CCP they kept this concept going they needed too to keep their small client base, Now by years of adding maps, they have a massive mapped system, copies of old maps with added backgrounds. its mythed by the inclusion of gates which are just the excuse of adding yet a another map. And they are about to do it again.
Rare developers that make large open world's for us to play in have gone the extra mile to create a true world with a true environment. that's an MMO to me and pure one.