Chrononauts is my favourite flavour of the moment.
Everyone can get behind the concept (you are time travelers trying to either steal artifacts or disrupt the timeline) without feeling too nerdy, you can explain the base rules in sixty seconds and handle everything else as it comes up, and after just one game you're pretty much as knowledgeable as you need to be to get strategic about the thing.
Seriously, I have never known a non 'throw-dice-to-move' game that is so pick-up-play friendly and gets everyone into the spirit of things (ie. screwing everyone else over) as quickly as possible.
As a bonus, I usually play with the house rule that if you're going to make a change in the timeline, you have to describe how you made that change. It makes amazing exchanges such as these:
"I place a timed-bomb in Hitler's chair at the games, thus meaning he never takes power."
"Well I open up the world's tastiest sandwich shop, distracting Hitler, meaning he never makes it to that chair."
"I place a timed-bomb inside Hitler's sandwich."
"I invent Subway and open a branch next to the sandwich shop, meaning Hitler chooses the Subway establishment instead and avoids the booby-trapped sandwich."
"I blow up your fucking Subway shop with Hitler inside."
and so on.
My one complaint would be that the events are skewed towards American history. I understand this, given that America has hardly any history allowing for a non-ridiculous sized game, but a Ancient Europe or Asia themed version would very much be snapped up by me. Despite this, the historical events are vaguely universal enough that everyone understands the timeline. Really, it's a niggle at best.
Best part - it's cheap.
£14.29 for UK folk and
$14.77 for US people.