Here are two I use frequently:

http://www.robtex.com/dns/ - The level of detail you can get from this site is voluminous. Check out the graph tab after a search for a cool diagram.

http://www.subnetonline.com/ - I use this primarily for the IP4 to IP6 conversion calculator for new windows 2008 servers when there is no IPv6 plan of action. At least this way there is some set value for servers based on internal broadcast domains. But there are soooo many ip tools here it can really do a whole lot more.