I tried a few free MMOs a few months ago.
I spent a week on RIFT and thought it was quite good, spent a few hours looking up HUD items etc (just like on WoW). Got it all setup & played it for a week but got really bored of it
Then I tried TERA and I think I played that for a month, really liked how it was a bit different and quirky, I really liked the control system.
I just got bored after a month, not really sure why.
I also downloaded neverwinter and played it for a few days
I had previously tried DDO and thought it was a bit dated
(if you don't mind paying for a game with no subscription) Guildwars2 is pretty good if you get into the control style but I just didn't like the way the weapons were setup etc

(hold sword or dagger and do long range attacks with it)
I was really disappointed with GW2 but only really because I hoped it would be like the original GuildWars (miss the cross building, skill tree, autoattacking etc etc)
I also gave EVE Online a go with a free 21day pass, I thought it was nice and different but as time went on large aspects of it seemed more spreadsheets online and I went back to X3 AP
Path OF Exile seems really good also (I have only really played it for 18hours)
I guess it depends on what you are into