It's called One, Two, Three. It's for two people (it can be adapted for more). Each person thinks of a word. When they're ready, they both say "One, two, three,
For example, if I said "cherry" at the same time you said "vanilla", I might say "Coke" next, and you might say "ice cream" (short phrases appear to be okay, as long as they are referring to one thing). Oh, also, you cannot repeat something, or have something that has too much overlap. So you could not have said "Cherry Garcia" next (although you could have said "Jerry Garcia"). The great thing is that there's no wrong answers, it's just whatever the two words make you think of.
Probably fairly easy to play over email. Might be fun and interesting to feed a bunch of word associations to a computer so that it could take a stab at playing this game.