NValidate 
Danger: rough draft ahead.
NValidate is a library for business logic validation. Validation, in this context, is a little bit like testing. But instead of trying to predict how the system will behave, it checks that the system has behaved in a certain way. It's useful for applications with complex and distributed business logic, to try to assert the simple truths underneath.
Consider a scientific bookstore management application. Your entities are books and authors. A book can have multiple authors, and an author can have multiple books. A simple constraint is that a book must have a title of more than 2 characters. A more sophisticated constraint is that each book must have some authors and each author must have some books. A validator for these would look like:
[ValidatorFixture]
public class BookstoreFixture
{
[ValidatorTemplate]
[Projector(typeof(Projectors.ForEachBook))]
public void BookHasLongTitle(CheckRecorder record, Book book)
{
record.That(book.Title, Has.Length.GreaterThan(2));
}
[ValidatorTemplate]
[Projector(typeof(Projectors.ForEachBook))]
public void BookHasAnAuthor(CheckRecorder record, Book book, IEnumerable<BookAndAuthor> booksAndAuthors) {
// booksAndAuthors is what you get by just reading in memory a many-to-many table.
var firstAuthor = booksAndAuthors.FirstOrDefault(ba => ba.BookId = book.Id);
record.That(firstAuthor, Is.Not.Null);
}
[ValidatorTemplate]
[Projector(typeof(Projectors.ForEachAuthor))]
public void AuthorHasABook(CheckRecorder record, Author author, IEnumerable<BookAndAuthor> booksAndAuthors) {
var firstBook = booksAndAuthors.FirstOrDefault(ba => ba.AuthorId = author.Id);
record.That(firstBook, Is.Not.Null);
}
}