These were the hottest items around in the Summer across all car forums. In a nut shell, you could pick up a 720p HD camera the size of a car fob for around £5 each delivered from ebay. It required a micro SD card ideally with 8gb of memory, but again you could pick these up for cheap money, approx. £12 each including delivery.
The footage they produced was outstanding for the size and price. Because of this, everyone was buying them for track days or even road journeys, mounting them here there and everywhere.
There's lots of variations of this camera so you have to watch out you don't pick up an inferior one e.g. Advertised 1080p ones are actually poorer quality than the 720p's but upscaled! THE best guide on these cameras is here: http://chucklohr.com/808/
I can confirm that I picked up one of the 'old' 720p version 3's. I can't say this will still be the case, but here's the ebay link to the seller I got mine from: linjiechong
Below is some road footage I took. The camera was mounted in the front grill under the license plate using sticky velcro pads. Remember, this video has been scaled down for youtube so the raw footage is even better ;)