For those unfamiliar with HFT / Algo trading this is a good, basic introduction, in the form of a video of a speech from Yan Ohayon at a TED function. It covers the speed of HFT, milliseconds down to microseconds, latency, colocation (I think it should be spelt co-location?), quote stuffing, and more.
Found originally at The Big Picture blog, here: http://www.ritholtz.com/blog/2011/04/the-impact-of-algorithmic-trading/
Also on YouTube, here: http://www.youtube.com/watch?v=4dEzNMx94s0