If you're not aware, the Game Boy Color has an infrared sensor at the top for wireless communication. It was so underused that Nintendo dropped the feature when making the GBA.
These games use it. Does anyone know any others? I was unable to find any sort of listing online.
EDIT: I'll update this list as more are mentioned
Bomberman MAX Blue Champion
Bomberman MAX Red Challenger
Harry Potter & the Chamber of Secrets
Mary-Kate and Ashley: Pocket Planner
Mission Impossible (thanks SparTonberry)
Super Mario Bros DX (combine your high score table with a friend's)
Pokemon Gold/Silver/Crystal (get mystery gifts)
Pokemon TCG (thanks Nionel)
Robopon (technically doesn't - it has its own infrared sensor built into the cart)
Xena: Warrior Princess (supposedly, but I can't find more details online)