I just used the site dl65 mentioned,
http://www.bandwidthplace.com/speedtest/, and I don't agree with it's results. Not accurate at all. I like dslreports.com. I just ran a report from them yesterday and got 1370 kbps download and 371 kbps upload. My rated speed is 1500/384. So, my actual download speed is about 91% of the rated speed.
After seeing this discussion, I tried http://www.bandwidthplace.com/speedtest/ out of curiosity; it reported 536.3 kilobits per second. I then immediately ran another test from dslreports.com and got 1371/370, nearly identical to yesterday's test.
dslreports.com also offers a Tweaks test and other tests. The Tweaks test provides recommendations, if applicable, on how to improve your speed; usually, this would involve changing the RWIN value in your system. Yesterday, I ran a speed test on my old Win 98SE system. The initial test yielded a download speed of about 870, as I recall. I then used DrTCP, a free utility from dslreports.com, to change the RWIN value, as recommended by the Tweaks test for the Win 98SE system. After changing RWIN and restarting the computer, I ran the speed test again and got about the same results as on my main machine, the 1371/370 mentioned above.