ive been playing mmo's since june 2004
ive played just about every AAA mmo title to come out since then, including some F2P titles.
A lot of the AAA titles that have come out since then have gone F2P..you have a great many options to pick from.
you can check out some of these (all F2P):
Star Wars the Old Republic Online
Age of Conan: Unchained
Aion
TERA
Lineage 2
Guild Wars 2 (it isnt F2P you just buy the game and theres no sub required, worth it imo)
Star Trek Online
Allods Online
There are plenty more F2P titles but thats where I would recommend starting. Someone above mentioned Path of Exile..its not an MMO. its just an ARPG like Diablo except its free to play. A very good game regardless, its currently in open beta, so you can play it. Its been heralded by some as the "true successor to diablo 2"
Games with subscriptions worth checking out:
Haters gonna hate, but 9.6million subs for a 10 year old game aint gonna lie...World of Warcraft
and RIFT
and my runner up...Warhammer Online (has the best RvR in an MMO thats out right now imo)
Also check out mmorpg.com and massively.com if you're trying to get into the mmo scene
MMO's in development that you might be interested in:
Elder Scrolls Online
Camelot Unchained
Everquest Next
Defiant
ArcheAge (out in korea, Trion picked it up as a publisher for an NA release, looks promising)
Lineage Eternal (im personally excited for this)
Neverwinter
Marvel Heroes
TITAN (the next hush hush blizzard mmo)