PhoneSnoop is a free, remote spying application designed for BlackBerry phones. The app works by intercepting phone calls from a predetermined 'trigger' number. When PhoneSnoop detects an incoming call from that number, it accepts the call and turns on the BlackBerry's speaker phone, effectively allowing the caller to listen in on the target's surroundings.
The Washington Post's Security Fix blog notes you have to have access to the victim's phone to install PhoneSnoop, which limits who you can spy on, but it still sounds like a great gift for jealous couples.
The programmer who wrote the app said it could be bundled with other apps and sneak on to a user's BlackBerry unsuspected.
So keep close tabs on your BlackBerry -- and what you put on it.