The tumblr XYXX is a "visual conversation between two iphones and two lovers." Essentially, this couple started sending photos back and forth, any old thing from their days, more abstract and less cheesy than you might expect, and they turned it into a tumblr as a record of the "conversation."
You can tell which photos were uploaded by him and which by her given their small tags at the bottom-- either XX or XY.
Though the photos are interesting, it's the concept more than the photos that I like. Here's what they say about it in their "authors" section of the site:
where conversations can be routine, an image is so much more intimate. it’s letting someone in behind your eye and saying right now, this second, this is my world. this is what i see, how I feel, and what I want to share with you. somedays it’s a game of exquisite corpse. somedays it’s a quiet conversation. An ever evolving game of tag with no rules or expectations except to simply be present, and that’s the beauty of it.