My two favorite games:
Words with Friends (aka Scrabble under a different name)
Scramble (like the Facebook game, or the old game Boggle)
Both let you play/challenge your friends, or play with someone random online.

Photoswap is kinda cool. You take a picture and it sends one back from someone anywhere in the world. You can have "photo conversations" which, of course, the gays have taken to a new level.
Mover is a cool app, but seems kinda buggy. You've probably seen it in an iPhone commercial. It allows you to connect with people sitting near you and slide photos from your phone to theirs and vice versa.