Bandwidth Tester is a Firefox extension that I used to use all the time. Basically it downloads a number of files of different sizes to gauge the speed of your Internet connection. This is particularly handy at coffee shops, where you don’t really know what kind of connection they have, or whether it’s worth using.
I quit using it because the guy who made it removed the files it used for testing, and it made Firefox lock up. Sad times.
There’s a new company starting up though (they actually went live last night) called RoundTwo, and they plan on being a source of Firefox extras. The guys that run it have done this sort of thing before, but they’re building on past experience.
The new Bandwidth Tester uses a distributed system to download test files, so they pull from a variety of places, lowering the impact on a single server. You can make your own files (which was a feature of the old one, but didn’t work), but I like the idea of pulling them from all over. It creates a more realistic Internet experience test.