Yes, that's a topic I wanted to look at too
Thanks.
OK, it seems there is an abundance of portals... In order to reduce the numbers I excluded all below PageRank 6 (I know, it's a lame method, still it gives some indication of portal's popularity) and came up with this list:
http://store.steampowered.com - Steam - probably the biggest one, rumoured to be the hardest to get into (get accepted)
www.desura.com - Desura - a lot of free games, can run alphafunding campaigns, seems like a developer friendly portal
www.onlive.com - OnLive - OnLive is a total proprietary API system. You must get accepted(which I believe is relatively easy...easier than Steam, harder than Desura) and then fully integrate their API(custom loading/saving system, custom config file locations, game needs ability to sleep/resume like mobile platforms, etc.)
www.bigfishgames.com - BigFishGames - targetted totally on casual players, all games have demo version which are always full games with 1 hour limit, all games have splashscreen with "BigFishGames" logo which seems like a requirement.
www.arcadetown.com - Arcade Town
www.pogo.com - Pogo
www.gamehouse.com - GameHouse
www.shockwave.com - Shockwave
http://zone.msn.com - MSN Gaming Zone
http://games.yahoo.com - Yahoo Games