Ace Online aka Space Cowboy aka Air Rivals aka... whatever...
MMO Flight combat sim with ground and space missions.
URL should be ace.subagames.com
It doesn't feature a monthly fee or a box price, yet revolves around an item shop mechanic where you are supposed to buy "goodies" for real money.
As most Korean "style" MMO games... gameplay is heavily focused on repeating quite simple tasks ad nauseum as well and is much more blatant and obvious about it than any of our "mainstream" MMOs,
which of course makes spending real money to help progress a little faster ever so more "attractive".

Not really what i would even consider a "free game"... and certainly not a "shining example" LOL.
Takes a lot of time to level up though. Best if you've got some friends
to power level or fight as a pack with. Once you get north of lvl 50 you can
deal a lot of damage.
Yeah... takes a lot time of monotonous killing basic enemies so you are able to do more and more damage to things that have more and more hitpoints -
giving you some shallow illusion of progress in what is essentially little more than the human version of a hamster wheel /pukes.