Dungeons and Dragons Online, a mostly forgotten game by anyone I know. Truth be told I found it a lot more enjoyable than when I first played it when it became free. So much so that I decided to actually buy some features. The trick is... learning the tricks in the first place!
The game is free to play, and despite what others may think, you most certainly do not need to pay a thing to hit level 20 and reincarnate. While it is true you can buy things to increase your power, it does not approach "pay to win" levels of brokenness. Should you decide the game is worth spending money on, I recommend a discounted Menace of the Underdark (which is available right now if you know how to do it, or wait for the rare Steam sale).
There are a LOT of the cash shop items that try to bleed you dry. Some seem reasonable to me, whilst others hell no. Seriously, $12-$15 for experience items on a single character? For reference, farming in-game currency is roughly 25 cents an hour at best... which is kind of a poor way to spend one's time.
Despite being a MMO, all quests are instanced. This means that the only time you will ever run into people is in towns. In a way it's nice, but it also feels empty for soloers in that regard.
It's quite possible to solo your way to 20, which is what my first character did. If you're a soloer, don't be afraid to hire some hirelings, especially healing ones. They are dirt cheap and they don't rob you of any experience at all. They do cause more monsters to spawn like what would happen in a group setting, but it's usually worth the increased survivability. For a rough time estimate, it took me a solid month of semi-dedicated playing and no knowledge of any quests to reach 20. With actual game knowledge, I could probably cut that time in half. With a good group, you can probably cut that in half yet again. And now there's a built in +20% daily completion bonus that should help speed things up.
The Action point/Enhancement system recently got a revamp and went from horrendous to kind of cool. Out of all the free MMORPG's these days, this is the only one I spent money on.