The Kensington WiFi Finder is relatively large (credit card sized) and scans before displaying the results instead of a constant active scan--but it can tell Wi-Fi from junk. WFS-1 from Smart ID is much cooler offering a no-industrial-design-intended box with instant scanning--but which "hears" all 2.4 GHz signals the same.
Push the button on the WiFi Seeker and it scans briefly, often under a second, before displaying a signal strength in zero to four LEDs (movie above requires QuickTime). Keep the button held down and it's a Wi-Fi dowser, allowing you to move around and see immediate response to different signal strengths. It's more directionally sensitive than the WFS-1.
Chrysalis will sell the device starting in June for $29.95, but you can sign up now to be notified when it's shipping. Earlier, PC Tel has said they would make the device available to their customers, and Chrysalis will sell custom branded versions of the device.
A reader who is a fan of the Smart ID WFS-1 wrote in to complain about my characterization of the WiFi Seeker as best in class. He's used the WFS-1 in hundreds of locations around the country, and considers its monitoring of the 2.4 GHz band (instead of just Wi-Fi signals) to be a plus, as he's learned to differentiate the pattern of flashing lights to distinguish from among Wi-Fi, microwave ovens, cordless phones, and other devices.
The WiFi Seeker displays solid lights to indicate Wi-Fi signal strength; the WFS-1 flashes or holds steady. I can see why this would be an advantage in mixed signal environments. For the purposes of my evaluation, I'm interested in determining only whether a Wi-Fi network is available. Case in point: at a hotel a week ago, I used the WFS-1 to see if there was an active Wi-Fi network. It flashed like crazy. After firing up a stumbling program and spending some fruitless time seeking the network, I realized the WFS-1 was showing my Bluetooth-to-computer connection. Perhaps with practice, I could have differentiated that.
I expect that a future generation of Wi-Fi detector will have the features that other readers have written in about: the ability to identify closed and open networks, and a display to show what network SSIDs (the Wi-Fi network name) were found, not just that a signal is present.