How can I implement NotOfType<T> in LINQ that has a nice calling syntax?

I am not sure why you don't just say:

animals.Where(x => !(x is Giraffe));

This seems perfectly readable to me. It is certainly more straight-forward to me than animals.NotOfType<Animal, Giraffe>() which would confuse me if I came across it... the first would never confuse me since it is immediately readable.

If you wanted a fluent interface, I suppose you could also do something like this with an extension method predicate on Object:

animals.Where(x => x.NotOfType<Giraffe>())

How about

animals.NotOf(typeof(Giraffe));

Alternatively, you can split the generic parameters across two methods:

animals.NotOf().Type<Giraffe>();

public static NotOfHolder<TSource> NotOf<TSource>(this IEnumerable<TSource> source);

public class NotOfHolder<TSource> : IHideObjectMembers {
    public IEnumerable<TSource> NotOf<TNot>();
}

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Also, you need to decide whether to also exclude inherited types.

This might seem like a strange suggestion, but what about an extension method on plain old IEnumerable? This would mirror the signature of OfType<T>, and it would also eliminate the issue of the redundant <T, TExclude> type parameters.

I would also argue that if you have a strongly-typed sequence already, there is very little reason for a special NotOfType<T> method; it seems a lot more potentially useful (in my mind) to exclude a specific type from a sequence of arbitrary type... or let me put it this way: if you're dealing with an IEnumerable<T>, it's trivial to call Where(x => !(x is T)); the usefulness of a method like NotOfType<T> becomes more questionable in this case.