LINQ to SharePoint is a custom LINQ query provider implementation for SharePoint that translates LINQ queries to CAML in order to query SharePoint lists. It comes with an entity mapping tool that exports the schema of a list and translates it into an entity type. Support to update SharePoint lists after querying them is planned.
LINQ to Active Directory implements a custom LINQ query provider that allows querying objects in Active Directory. Internally, queries are translated into LDAP filters which are sent to the server using the System.DirectoryServices .NET Framework library.