For coop NOLF2 is brilliant. I can't remember the player limit though.
Empire Earth was always great as a LAN RTS. And available on GoG. I tended to built lots of walls that would piss off many of friends.
The old C&C games might be something as well. I used to love Red alert 2, though I don't know how well it runs on newer systems/OS.
Thanks for the suggestions! I'll definitely look into Hidden: Source (never heard of it before...) and will add Empire Earth and Splinter Cell: Chaos Theory to the playlists. Of the other suggestions, I already had a lot of them, but it's good to see them mentioned again to remind me of them.
Anything else? :-D
We always play:
Left 4 Dead 2
Team Fortress 2
Dota 2/League of Legends
Then on occasion stuff like this:
Red Alert 2 (hard to get working nowadays)
I wish that everybody who suggested Supreme Commander was my neighbor for a quick and easy lan session whenever we have the time. We are too few.
Sven Co-Op is pretty great as a co-op campaign (Hl1 mod), else the Serious Sam games have a pretty decent co-op campaign.
My favorite games for local multi-player (Nintendo consoles/emulations aside) are:
SWAT 4 is an amazing LAN game. It works best with enough people to have two 3-4 person teams, but I've had plenty of fun with a single team of 4 or two teams of 2. Having multiple teams means you can start at multiple locations/entrances in most of the levels which is both more interesting and a big advantage. However, smaller teams will probably die so it's a trade-off.
Left 4 Dead (2) is one of the better 4-player co-op games out there.
Artemis is a wonderful Star Trek style Bridge Simulator game in which you and your comrades command a space ship. 4 is a decent sized crew--you can leave out non-essential personnel or have someone double-up on roles (doubling roles gets interesting if you pick two fairly complicated ones ...). Some of the roles can be taken on by multiple players as well (you can have a Helmsman/Science Officer and a Weapons Officer/Science Officer) which allows you to focus on multiple tasks at once more efficiently as a team. You could even each give yourselves a full ship and try not to die horribly. :) I seem to recall that there are community designed missions ... I forget if there are any non-random missions that come with the game, but we have the most fun with customizing the random/procedural mission generator. It's $40, but it is DRM-free and the creator gives explicit permission for you to share the game with your fellow bridge members--he just doesn't want you to distribute the game willy-nilly for obvious reasons. You can buy it for yourself and share it during the gaming session, or split the cost with your mates. In the FAQ he also claims to offer a refund if you're unsatisfied, but I don't know anyone who has tried to get one.
The Unreal series makes for fantastic LAN play as well.
I think of [the Internet] as a grisly raw steak laid out on a porcelain benchtop in the sun, covered in chocolate hazelnut sauce. In the background plays Stardustís Music Sounds Better With You. Thereís lots of fog. --tomeoftom