"Omerta" -
http://www.barafranca.com
"Ikariam" -
http://www.ikariam.org
"Tribal Wars" -
http://www.tribalwars.net
"Civ-Online" -
http://www.civ-online.com
"Lunia" -
http://lunia.ijji.com
I'm taking a good look at these and play 'em pretty regularly.
If you don't play "Omerta" often you'll end up killed.
With "Ikariam", "Tribal Wars" and "Civ-Online" you require lots of time and patience because you're building an empire. It can be boring, but something you can log onto once or twice a day to play.
"Lunia" is a 3D MMO Arcade/RPG game with okay-looking cel-shading and a few classes to choose from (the usual swordsman, mage, etc). There's a seemingly-vague storyline, a rather large following and it's not so bad.
I kinda like the idea of "Quake Live" and might play it. I have played lots of MMO games and even if they rarely live up to my expectations, they're alright for passing the time. It really is amazing how many FREE games that are both single-player and multi-player there are out there!
The problem with so many of these free games is this: you donate a little money (1 Euro or whatever) and that grants you far too many priviledges for the game to stay balanced.
One game that I hate is "Ragnarok Online", possibly worst MMO game ever but still so popular! Why??
"PKR" is fun to me, too, but I don' feel like wasting all of my money on 3D poker. It's free, yeah, but there is always the option to play for real money. It is sometimes annoying with all of the emotions and actions you can use.