NetStumbler is one of the oldest and most known Wi-Fi stumblers and runs on Windows and Windows CE/Mobile. It lists nearby APs and displays their basic details: SSID, channel, speed, MAC address, vendor, and encryption. Unlike most other stumblers, it also shows the signal, noise, and signal-to-noise ratio levels. Additionally, it has GPS support to record AP locations when wardriving.
Keep in mind, NetStumbler hasn't been updated since 2004. It may not run well on Windows Vista or 7, or even 64-bit Windows XP. Additionally, it doesn't show the real encryption methods of APs. If an AP has encryption enabled it's always marked as WEP, regardless if it's WEP, WPA, or WPA2.