From Mark Edwards:

This is a really slick innovation: The Linksys Wireless-G USB Network Adapter with Wi-Fi Finder (model number WUSBF54G). It’s a wireless network adapter with a built-in wireless scanner to detect available networks, and it works as a standalone unit so you can detect 802.11b/g networks before you power up your laptop. Having one of these is sort of like having Netstumbler in device the size of a pack of gum!

  WUSBF54G

Mark’s blog here.  Product link here.