A tool I’ve found enormously valuable if you are selling a web product is: http://builtwith.com/
You can type in a competitor’s name and it will show you every website on the internet that uses your competitors technology. It is really great for generating leads or finding out which industries / countries are under / over serviced.
For example http://bounceexchange.com/ adds a popup screen on your websites to decrease your bounce rate. Searching builtwith.com for Bounce Exchange (http://trends.builtwith.com/ads/Bounce-Exchange) gives lots of really good information.
Contributed by James Brotchie