What card games work well with a large number of players (say 6+ players) and only require a standard pack of playing cards? I'm particularly interested in games where the rules can be explained fairly quickly, but there is still a reasonable level of tactics. Also it would be preferable if the game can be played in a single room.
One game per answer please.
The best large-group card game that I know of is Up and Down the River, which Wikipedia knows as Oh, Hell. I find the rules given by Wikipedia to be confusing, though, and found a better set here at eHow. (Note that both of these rulesets differ slightly from the version I'm familiar with, as there are endless variants of most of these games.)
This is one of my favourite card games. You have to use inductive reasoning to figure out the dealer's secret rule. It's actually better with a large group because you learn more from other players' mistakes while waiting for your turn to come around.
It can be a bit intimidating for new players, so you might want to start with a small group playing Eleusis Express to get the hang of it.
If you have enough players (8+) you can play werewolf with playing cards:
King of diamonds == Seer
Ace of Spades == Mayor marker
Many poker variants, including the very popular Texas Hold'em, can support a large number of players. In Texas Hold'em, each player is dealt only two cards of their own, there are 5 community cards, and 3 burn cards, meaning that you could in theory support 22 players if you deal out all of the cards, though generally you play with 10 or fewer players on a table, maybe up to 12 or so.
It does require something to bet with—chips, pennies, stones, dried beans, or any other form of counter—so it can't be played with just a deck of cards, but usually it's pretty easy to find some kind of betting counters. If you don't like gambling, you can play it as a tournament, in which each person gets a fixed number of chips and the last player left with chips wins.
You can find lists of card games for different numbers of players at pagat.com. In particular, Looking for Friends (找朋友) looks interesting, though I haven't tried it. It's described as a trick-taking game that works well with large groups.
Hearts (yes, similar to the Microsoft game) works surprisingly well with 8 people and 2 packs: I've not tried with more, but there's no theoretical reason why not . Racing Demon (one pack per person) is also good, if a little raucous with high numbers.
Mao can work well with large numbers of players, or you can open a few more decks and get several different games going in the same room. It's also an intriguing game to watch, for those who don't understand how to play.
Creights is a very engaging, Uno-like party game played with a standard deck. It is about 70% luck and 30% skill, in my estimation.
It scales easily to about 7-8 players, but requires two decks shuffled together with 5/6+ players. It doesn't require those two decks to have the same backs, but make sure to remove a couple of 5s from the deck if you play with two decks.
