I always liked gfblip: http://gfblip.appspot.com/

You can run this on your laptop and watch your WiFi drop in and out randomly (as other people around you download stuff, as Bluetooth devices interfere with it, etc.) It's also fun to run a speed test in another tab, and watch the latency increase 10x as needless buffers fill up and your latency-sensitive packets wait in line so your router gets a better review!

I found that tool from Ben Kuhn's blog, and at first it seemed great. But I've seen some weird behavior from it. Examples include drastically different behavior on a refresh (and not just for the 'blue' website, also for gstatic.com) and pings to Madagascar which are apparently faster than the speed of light. So now I'm somewhere between confused and distrustful.

If anyone has a detailed guide on using gfblip I'd be really curious to read it.

Check out the instructions in the GitHub repository, has two versions, neither are very long nor complex. https://github.com/apenwarr/blip