netcraft  
 Gephi

Maltego

Maltego does all the work for you, like a train of thought that keeps going around on by Maltego pre-laid tracks. That may be acceptable in some cases; but IMHO it always pays off to know what's going on behind the scenes. Much of its operation is abstracted from the user, and sometimes it can be hard to tell what is actually discovered. And in a way, it just provides a wrapper (that you have to register for on their site, and for the more complex operations pay for) around the more simple tools listed here. When using those, and understanding their use, you keep thinking new thoughts and by going off the track you are more likely to find those extremely useful morsels of information that make all the difference.

netcraft  
 Gephi