I've come across some really useful tools which were hidden rather deeply in Google's portfolio of useful stuff!

The first is the research function in Google Ad planner - you can get a detailed breakdown of your competition - who's looking at their site, the sites that people that visit their site also go to and keywords that are used to find their site. Its possible also to compare competitors. It's definitely worth the sign-up.

With any such competitive intelligence tool you need to use sensibly and to understand how the data is gathered but its a great free resource.