Left 4 Dead is only on 360....for now. I'm predicting there will be a L4D collection on the PS3 before the end of the year. And as far as online goes, the 360's only advantage is the overriding chat. The games play exactly the same, there's zero difference. I can personally attest to this for Medal of Honor and Call of Duty watching my cousins play them for many consecutive hours with no issues at all. Also, the PS3 is a better all-purpose machine than the 360 and a helluva lot more reliable. And you don't have to pay extra to play half the game you just bought which is a big plus for a lot of people (not just me).
As for the games with online (put it point form so it's easier to read):
- Uncharted 2 has some multi (and co-op missions) that were OK
- Castle Crashers is an obviously downloadable title for multi, it came out on PS3 last year
- if you like FPSs, Killzone 2 is supposed to be pretty solid online and I can personally attest to Medal of Honor and Black Ops being "very good" according to my cousins who play online
- Modern Warfare 2 has the Spec Ops co-op mode which is FAAR superior to the run-and-gun online IMO and almost worth the price of admission on its own
- Borderlands is a great one if you want a Diablo-style FPS that's more about exploring and the drops than not having a life and earning "perqs"
- not sure if the Lego games are playable online or not but if they are, they're excellent (particularly Star Wars and Batman) and feature drop-in, drop-out multi which is great
- 1942 Joint Strike (downloadable) is playable online if you like a shooter
- online play was the big draw in Marvel vs Capcom 2 (and the upcoming #3), which is a downloadable title
- don't forget about Street Fighter 4 if fighting games are you thing
- Burnout Paradise is a phenomenal racer with tons of online options as is Gran Turismo if you prefer the sim racing
- Rock Band (2 or 3) would be a great online music game if you're into that, far superior to Guitar Hero, particularly for people who've actually played music in the past.
- Resident Evil 5 is much more fun co-op than on your own and it has some online vs options as well. At less than $20 now (used) it's a great deal
- the online options are a great excuse to pick up Bioshock 2, which is $20 new or about $15 used. And since #1 is under $10, you can't pass it up too even without online. That way you'll get to play through the awesome single-player games
- as a programmer, you'll probably appreciate Little Big Planet more than most with its level building and sharing
- the Marvel Ultimate Alliance games (there's 2) are a great Gauntlet-style beat-em-up that are fun as hell and pay great lip-service to the Marvel fans
- and of course there's Red Dead Redemption's Undead Nightmare pack which has all of the online expansions in a stand-alone disc (kinda like the Episodes of Liberty City Stories from GTA4). You don't even need the original game but you should still get it anyways, it's that good
Others can let you know whether or not Assassin's Creed Brotherhood is any good online (or single player; #2 is excellent and cheap now but no online). Coming out in a couple of months will be Earth Defense Force: Insect Armageddon. If it plays anything close to the 360 one, it will be an awesome online game. And it will be 4-player co-op so when you decide to getting around to adding some of us to your friends list (PSN & 360: Zeusy1976) you can have even more fun :-)
As for some non-multi games, here are my recommendations:
- Metal Gear Solid 4 (tell me you didn't know that was coming) - I believe there's some multi in this as well but I never noticed
- Castlevania: Lords of Shadow (must-have IMO, WAAAAY better than the critics say)
- Demon Souls (if the servers stay up)
- Valkyria Chronicles
- Fallout 3 Game of the Year Edition
- Mass Effect 2 (it comes out in a few months with all the expansions)
I was there on that fateful day, were you?