An article in Wired points to a US government web site broadband.gov which has a test (as a Java plug-in) that tests your broadband speed. The Federal Communications Commission is going to use the results of the test aggregated across large numbers of users to find out the real effect internet access speeds people have - as distinct from what their broadband providers say they have. The site asks for your address so as to correlate speed by location.

The site is very slow at the time of this writing. My guess is a lot of people found out about it at once and it isn't built to scale. Let me know in comments if you have been able to use it successfully and what speed you get.