Authenticating web app users against Active Directory
The article is complete and easy to understand, but misses to mention 2 crucial points. Within Login.aspx, you need to specify "Imports MyApp.FormsAuth". This will import the FormsAuthAD Namespace, otherwise declaring a variable of type "LDAPAuthentication" will result in a compile time error.
Also, if you cut & paste the code, be sure that any references to "FormsAuthAD" are changed to the name of your project. This will ensure you import the correct class from the correct assembly. Aside from these points, the article is nicely written and easy to understand.