speedtest.net, does sampling and drops the slowest 20, and fastest 10, and returns the best remaining sample. Thus more likely to show a peak speed.
We do an average across time, so if the PC is slow to do a TCP ramp it can affect ours, as also can variable speeds from the provider.
speedtest.net good for pub bragging
thinkbroadband.com good for knowing what non HTTP downloads will be like
Traffic management has a tendency to impact our test as its not HTTP based.