http://www.logler.com/2006/04/12/casual-games-portals-major-list-second-edition/

AOL Games Channel - http://onlinegames.channel.aol.com
Arcadetown - www.arcadetown.com
Bigfishgames - www.bigfishgames.com
Boonty - www.boonty.com
EA’s Pogo - www.pogo.com
Gamefiesta - www.gamefiesta.com
Gamehouse - www.gamehouse.com
Gameslammer - www.gameslammer.com
Gametrust - www.gametrust.com
Gamextazy - www.gamextazy.com
Gamezone - www.gamezone.com
Garagegames - www.garagegames.com
Grab.com - www.grab.com
Hotgames - www.hotgames.com
Midasplayer - www.midasplayer.com
Miniclip - www.miniclip.com
MSN Gaming Zone - http://zone.msn.com
Nextarcade - www.nextarcade.com
Oberon Games - http://gamecenter.oberon-media.com
Playfirst - www.playfirst.com
Popcap - www.popcap.com
RealArcade - www.realarcade.com
Realore - www.realore.com
Reflexive - www.reflexive.com
Shockwave - www.shockwave.com
Terragame - www.terragame.com
Totalgaming - www.totalgaming.net
Trygames - www.trygames.com
Tucows’s Playclique - www.playclique.com
WildGames - www.wildgames.com
Yahoo Games - http://games.yahoo.com
Zylom - www.zylom.com

These were mentioned in the reply

Illusionsoftware - http://www.illusionsoftware.be
Absolutist - www.Absolutist.com
Gamehorde - http://www.gamehorde.net/
Casual games are those simular to "Tetris". However there are other types of games hosted on these sites. Thought I would share this list with everyone, if you know of some that are not on here then let me know.