Cards Against Humanity
The Resistance
Pandemic
Skull
Zombie Dice
For adults now!
I am a fan of 'selectively curated' versions of Cards Against Humanity, aiming to keep the humour on the right side of potty. Cards Against Humanity (a nasty game for nasty people) is one of the most interesting and innovative games created in the last decade. It's origin, development, and continuing story is fascinating (see the Wikipedia article). It is a game for both large and small groups, and really cashes in on 'clever thinking' of players matching 'responses' to the 'prompt'. Very very rude and deeply offensive, however I have crafted a 'Irish Mammy's' version and a friend has also made a 'University not so PC' version.
Another game I like is Pandemic. A cooperative game, you play against the system and very difficult to win. It says 2-4 players but more can take part as players tend to discuss and strategise, and because you're all on one team really, fighting against viruses and plagues that relentlessly multiply and spread around the world. This game is a modern classic. It takes about 45' to play through, after which you want to play it again again as you delude yourself into thinking "next time I won't get my ass whipped".
The next are two of my favourite casual games that can be carried around in the bottom of a backpack and brought out in a lonely pub on the Western fringe of civilisation with an Atlantic gale proving that water falls upwards.
Skull -
Zombie Dice -
Both amenable to drinking punishments.
Oh, and The Resistance.
If you like Werewolf and hidden identity games then the Resistance is one of the best new reboots of the hidden identity mechanic. It also scales well to larger groups.