signed_request parameter is used by the framework. From October 1st 2011 all Facebook Apps and Facebook Pages must use this parameter to get information about the current user, page id, access token etc.
At the time of writing, it seems like it is only the offical Facebook SDK at CodePlex which is satisfying all the requirements when it comes to Facebook Development in .NET.
Unfortunately, the SDK contains a lot of bugs, and is very hard to debug if you happens to find one.
Personally I was forced to write most of my Facebook code from scratch without using the Facebook SDK due to all the bugs.
You can find the documentation of the Facebook API at the Developer area.