Star Trek Online - I really like the ship combat. It does get a bit repetitive after a while, so I take breaks to reset mentally.

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.

Warframe is in my favourite. Be a space ninja doing flips while electrocuting a whole room of enemies before blowing them up with your hand carried cannon. Also fair monetisation and a very friendly community.