Geez I'm going to sounds a little out of place......
It's hard to go past a good old fashioned french onion dip! A Classic People!
however that gets old quickly, so you have to have back-ups. Personally i LUUUURVE hummus. But not your pre-packaged crapola from the supermarket. Like, freshly made hummus from a (decent) turkish restaurant (all the best ones let you buy some to take with you anyway). And I find it delicious not only on Turkish bread, but also Turkish Pide, Kebabs, chips, biscuits, sandwiches - anything savoury really
Guac pretty damn good - especially on nachos
Straight sour cream - that's a bit bland - yummy, but fairly simple in taste - need a little something else with it
Not sure if you international guys have these or not, but we have these things called sweet chilli philly (phillidelphia is brand name) which is basically cream cheese with sweet chilli sauce over the top (you tip it upside down to serve - oooh novelty factor). Needless to say its delectable.