Are you sneaky enough for the SIM Card Spy Gear Remote Listening Device? Just take your SIM card out of your phone and stick it in this mysterious black box that's about the size of a bar of soap. Hide the $85 device in an inconspicuous location wherever you want to do your listening, and then when you call your phone number from another phone, you suddenly have ears in exactly the right places. Please, use this for amusement only, you busybody. [Brando, via OhGizmo]
AU: I recall from a spy-related TV show on Foxtel that almost any handset can effectively do this. Switch it to 'silent' ringer and turn on 'auto answer' and you're away. But use only for good, not evil.