Great unimposing games

Kakerlakenpoker Royal (or basic kakerlakenpoker).

Basically you lie your ass off (while trying to load other players with insects and other critters) and that's the game. This is the game I just have to promote. The mostly lying with minimal rules.

I personally like speed recognition games (which usually scare most gamers away) - Geistesblitz is a really tough one that had eurogamers screem "oh, this it too THINKY" (can we play Agricola instead?). Especially if you add Geistesblitz 2.0 into the mix. With kids I game with (8-12 y.o. in a social youth centre) Jungle Speed is highly popular. I add some expansion cards in the mix to mess with them and usually beat them. And if you want something more uhm thematic (har har) there's Pick a Pig... (what? it looks cute)

Other games these kids love: King of Tokyo and Cash and Guns (yes, they're this kind of kids). But most love gets Pyramid . I did play it with adults as a kind of filler, session closer or a midnight game (uhm, don't ask, we also played Animal upon Animal at 2 AM and thought it was the shit).

Card games are great and often unappreciated, check these: Divinare (okay, kinda silent, but it's a double guessing, manoeuvring sorta game), Mamma Mia (yes, yes, kids game, who cares, I make pizza in a game!, get off my lawn), My little Prince (tile drafting which is also mean if you're an eurogamer, if you're not it's also pretty cute),
Chiming in with what other's have said: Tichu, Haggis (superb, like it better than Tichu), Condottiere, Citadels obviously. I also enjoy Havana (kinda like Citadels but with less killing and more elbowing - tends to be too expensive for what it is though, unless you buy a Dutch version like I did)