SimplyGold

Show all domains that your computer belongs to:

public static void ShowMyDomains()
{
    DomainCollection domains = Forest.GetCurrentForest().Domains;

    foreach (Domain domain in domains)
    {
        Console.WriteLine(domain.Name);
    }
}

Show all users in the domain:

public static void ShowDomainUsers()
{
    DirectoryEntry domain = new DirectoryEntry("LDAP://domain.com/CN=Users,DC=Domain,DC=com");

    foreach (DirectoryEntry child in domain.Children)
    {
        Console.WriteLine(child.Name);
    }
}

Show all computers in the domain:

public static void ShowDomainComputers()
{
    DirectoryEntry domain = new DirectoryEntry("LDAP://domain.com/CN=Computers,DC=Domain,DC=com");

    foreach (DirectoryEntry child in domain.Children)
    {
        Console.WriteLine(child.Name);
    }
}

Resources

1. LDAP on Wikipedia. It read like Latin.

2. How to do almost everything in Active Directory via C# from the ever-excellent Code Project. Reads like Pig-Latin.

3. Active Directory Authentication from ASP.NET from MSDN.