Recording a conversation on your smartphone isn't as easy as you think it would be. Varying state-by-state laws make the legality a bit of a gray area, and if you don't have access to the free Google Voice service, you're going to have pay a monthly fee to capture a call. Or, put up with the U2 Mobile Recorder hanging off your handset.
In a round-about way the $134 U2 Mobile Recorder simplifies the act of recording a call on your iPhone or Android-based device. You just plug it into the top-mounted headphone jack of any phone except the iPhone 5, and hit record. Four gigs of non-expandable storage should be able to hold up to 144 hours of conversations, and a USB port allows the recordings to be dumped off to your PC. It's also got its own headphone jack for immediately transcribing a conversation, and on a single charge it can record or play back for about 20 hours. Which is perfect for even the most long-winded of snitches. [Amazon via 7Gadgets]