Shieldlist.org is searchable database containing information about more than 200 Arduino shields. Every shield has its own page with short description, link to its homepage, licensing and source information as well as very useful diagram of used pins. more features are planned in the future. If you submit new shield you can get a Freetronics ProtoShield Basic.
Image source (John Boxall CC BY-NC-SA 3.0)
The shield list is a great resource, and has saved me a lot of time when mashing up my shields. It also helps you in a quest for a certain type of shield. Great work!