The Olympus TP-7 Telephone Recording Device (clever name, eh?) is probably the easiest way to record phone calls (from an abusive debt collector, for example) that I have seen. You stick it in your ear, plug it into a recorder, hold your phone normally up against your ear with the earbud in it, and it picks up both sides of the conversation!
You can use a tape recorder, but many MP3 players will also record, so you may not need anything more than what you carry with you already.
Quick. Easy. Good stuff.
Olympus TP-7 telephone recording device | BoingBoing
