Ok, this is the coolest app I've seen in a while.
First, some the back story. I'm a network geek and have fiddled with plenty of Linux-based firmwares for the WRT54G/GS/L wireless routers. I've actually build custom installs from Sveasoft firmwares, tried OpenWRT but finally settled on DD-WRT as my favorite. I literally have five of these routers laying around. My entire house is nothing but a giant wireless bridging network with a few dd-wrt routers and a Cisco 871w.
And it's important to note this application is only available (as far as I can tell) in the "Standard" edition (std). Some newer revisions have less hardware, so they can only support the micro or mini versions. So if you have an older dd-wrt (ie rev 2) or the WRT54GL, you're probably OK.
It's called Wiviz for wireless network environment visualization, and well, it does just that. Imagine a swirling vortex of the local wifi devices!
If you own a dd-wrt enabled router, you already have this app, under the "Status" tab at the top then "Wireless" tab at the bottom. At the bottom of the page, where you'd normally see the "Site survey" button, there's another button, "Wiviz survey". My dd-wrt micro editions did not have it.
Check it out! Even if you don't use it on the router, it's a neat app to try on your wireless workstation.