The Kensington Wifi Finder is a really handy little gadget. It sits on your keyring and if you ever want to know if you have a wireless signal, just take out your keys and check the LED indicator - which certainly beats the time and effort involved in booting up your laptop to see what hotspots are available.
It should be able to filter out other wireless signals, such as cordless phones and bluetooth, and is able to detect 802.11b and 802.11g networks. Obviously it's a slightly blunt instrument and you may well pick up more hotspots from a further distance with a stronger antennae on your laptop, but it's certainly a decent indicator. You can get these from a few different places, and the quality is much the same regardless of manufacturer. This one from Kensington retails for about £20 at Amazon.
Verdict: Recommended gadget for Wifi fans.