This is a great site for learning about pretty much any domain or IP address:   

    http://www.dnsstuff.com

This is a really great site for a visual trace route:                                               

    http://www.visualware.com

Some great info on website traffic:

    http://alexa.com/#

AS, or "Autonomous Systems' and network relations, or what networks are connected to what network:

    http://www.fixedorbit.com

These three sites are great for reverse phone look-ups, NPA-NXX lists, look-ups and maps, Area code look-ups, lists and maps, as well as LATA lists, information and maps.

    http://www.reversephones.com

    http://www.fonefinder.net

    http://www.nanpa.com