In my previous post I showed the LINQ way to cast List<SomeDeriveType> to List<SomeBaseObject>.
As @jamiei commented this will raise an exception if the cast fails. OfType<T> will return only the elements of type T despite the fact that you have different derived types in one list. So suppose you have an Animal class and a Cat and Dog class that derive from Animal you could do something like this:
List<Animal> animalList = new List<Animal>();
//Get the dogs
IEnumerable<Dog> dogList = animalList.OfType<Dog>();
//Get the cats
IEnumerable<Cat> catList = animalList.OfType<Cat>();
LINQ makes it very easy to seperate the Dogs from the Cats!