iPhone: Apple's got a great app that rings or tracks your lost iPhone using its location data, but you can only use it if you've shelled out $119 a year for Apple's lame-duck MobileMe service. TekTrak does the same thing for $6.
We've mentioned Find My iPhone before, and it's great if you actually want a MobileMe subscription, but if not, $119/year is a lot of extra to pay for lost iPhone insurance.
How It Works
For $6, TekTrak runs in the background, using your phone's location data to track your location based on user-defined intervals (you can set it to check in every 10 minutes, 20 minutes, etc). If your phone's been misplaced or stolen, just log into the TekTrak website. From there, you can map where your phone was last time it checked in, try to ring your phone (in case it found its way into the couch cushions), or set the site to locate your phone, which will set your location to continuous mode (providing up-to-the-minute location).
Where It's Lacking
You can't argue with the relative price for the features you're getting, but TekTrak can't quite match up to Find My iPhone in a couple of ways. For one, it needs to be running in the background to work (for the non-jailbreakers, that's 3GS and above). Also, Apple's Find My iPhone can ring your phone even if it's set to silent; TekTrak can't (that's an iOS limitation). Finally, TekTrak can't remotely set a passcode or remotely wipe your phone. A nice solution for the first problem: just set a passcode beforehand.
At the end of the day, if you're not willing to shell out for MobileMe (and we don't blame you), TekTrak looks like a decent alternative. While you're shopping for alternatives, you may also want to check out the similar Undercover ($6) or FoneHome ($2.49).