Here's a collection of some of my favorite sites for work. If you have any to suggest post a comment and I will check it out.
This site has a collection of cisco configs with great explanations. Ken Boone of Boone Networking Services runs this site. He has been a great friend and mentor over the years. Thanks for sharing the knowledge!
Simple little site that will let you know what your external IP address is.
Site info, reverse IP lookup, domain info, links, ratings, etc.
You can type in an IP address and it will locate it for you on a map.
The "Swiss Army Knife Internet Tool". Very handy site.
Nice and clean online subnet calculator
Another nice tool for finding info on an IP address
MAC Address Lookup
Simple site to find the manufacturer of a MAC address.
One of the best pre-made security packages out there. If you have never used Back Track you need to check this out.
This site offers some of the best freeware and trial-ware utilities in one spot.
CCIE Voice Notes
A great blog for anything related to Cisco voice.
Very simple website to check almost anything email related. MX records, Blacklists, etc. Thanks for suggesting this Becky.
Test Phone Numbers
Here's a link to a site listing test numbers. How many times have you needed a number to call on the PSTN that would just answer with an auto attendant? Here you go!