Star Wars: The Old Republic
It's not as expansive as many MMORPGs are for dungeons and raiding (flashpoints and operations) unless you subscribe and join an active guild. However, the RPG storylines with player choices are fabulous.
The base game is free and there are a ton of activities to participate in without having to spend any money. Events rotating on a regular basis, PVE, PVP, start-fighter action, or just play solo and explore a ton of planets and storylines. If you want to dip your toes further into the expansions, subscribe for one month and they all become available permanently.
Definitely worth a try for someone looking for casual entertainment with end-game playability if one wishes to do so.