Ultima Online is a 25 year old MMO that still has the original servers running. The install is about 3GB and can run on any PC from the last 20 years. For the official servers, the player base is largest on Atlantic and you can sign up for a free Endless Journey account. PVP is only allowed in certain zones.
There are many many player run servers that fit a lot of play styles. The most populated being Outlands. That is where I mainly play as it is by far the most populated UO server, 2.5k-3k people on usually. Just beware, outside of the starting zone and towns, it is open PVP almost everywhere.
City of Heroes now has player run servers. The install is around 5GB. Homecoming is the most populated, with a lot of added content. Rebirth is less populated but tries to be the closest experience to the official servers.