I downloaded a jigsaw puzzle game the other day called Puzzle Arcade. I haven't spent much time with it yet, but I know I will... especially since it's something my non-gaming family can enjoy when we get together during the holidays. One of the game's cooler features (which I can't use yet, since I don't own an Xbox Live Vision Camera) is being able to upload your own images for puzzles. Apparently, you can't save those custom puzzles if left unfinished, but it's still cool.
I love the idea of players introducing unique, personal content in such a simple way.
I've said before that MMO designers should move screenshot viewing in-game.
Audio integration, such as enabling players with mics to record brief clips for character use (warcries, taunts, jokes, etc), is another possibility.
Another example is linking game audio to a gamer's mp3 library so that particular player-selected mp3s are triggered by particular events. It could be as general as this song for combat and that playlist for driving. Or it could be as specific as playing this song when I equip my two-handed maul, playing another song when I equip my bow-and-arrows, another song when I hop on my horse, another when the sun sets, etc.
Face-mapping isn't easy, but seems possible. Allow players to upload a photo of their own faces from a particular perspective. Software finds certain points on the face (nose tip, bridge width, eye spacing, etc), and then recreates the same characteristics in the character creation system. Note that I'm not talking about overlaying a character model with the actual photograph. I'm talking about using a photo as reference for a virtual representation using a creation system like that of Everquest 2 and Saints Row 2.
These personalizations require minimal effort from the player and don't require active input or feedback from developers (past initial production, I mean). What are some other possibilities?