Stealth Signal, available now, is a solution that helps track down stolen Macs.
The Mac owner simply installs a small, undetectable program (Stealth Signal Transmitter) on the Mac. When installed, this will send a signal to the Stealth Signal Monitoring Network at random times. When the Monitoring Network receives a signal it records the unit's ID and the time of transmission in its activity records database. These records can then be accessed (by registered owners only) online.
If a Mac gets lost or stolen, the Stealth Signal Transmitter remains active, and a Mac's owner can track down the computer by informing Stealth Signal. The security service will then track down the Mac by ISP.
Mac OS 9.x and OS X are supported by the solution, which also supports a variety of Windows systems. A year's service costs $35 for a desktop Mac, or $45 for a portable.
There's a comprehensive review of the product on SecureMac.com.