You guys who are posting how they are all parasites, how they contribute nothing, are just flat wrong. You don't know what you are talking about. Let me give you one example: the spread. In times before high speed trading, the spread between the bid and offer might be quite large. One effect of HFT has been to reduce this spread to almost nothing.
Why does that matter? Well, if you buy a stock, and then decide to sell it, you will pay the spread. It's like buying a car. You drive it off the lot, the price goes down by 20%. HFT has created a situation where you can turn around and sell that car for 99.999% of what you bought it for. This helps ALL investors.
Wide spreads also give brokers more of an opportunity to mess around. They can put orders on both the bid and offer, wait for them to get hit, and get paid the spread for doing nothing.
The truth is technology has totally opened up the trading world to a wider group of people than ever before. For a few hundred dollars a month, you too could have a server colocated with the exchanges. You just have to have the balls to try. In the old days, such access was only for the select few. Stop bitching about progress, especially technological progress. I thought this was slashdot?