Here are a number of links to sites we use or think you might find useful:
General
WhatIsMyIPAddress.com - Useful for finding your external IP Address if you are behind a NAT router.
http://checkip.dyndns.com:8245/ - Alternative site for finding your IP address. This one uses port 8245 to bypass any proxy server that your ISP might be using. By comparing the address returned from WhatIsMyIPAddress.com and this site you can also confirm whether your ISP is using a proxy or not.
Network Management and Testing
CentralOps.Net - Various network testing utilities such as Whois, NSLookup, Domain dossier.
Whatsmyip.org - Includes a test that checks for open ports remotely.
http://www.dyndns.com/support/tools/openport.html - Useful for checking if a specific port is open.
Network Security
Network Intrusion.co.uk - site with links to all the most important security resources online with comments. Run by an ex-military 'pen tester'.
Offensive Security - Developers of BackTrack (a linux based security auditing tool).
Broadband Speed Testing
Think Broadband - Virgin Media carried out testing of all the major 'broadband speed testing' sites and this was the only one that gave accurate results. See the news article here. Also compares various ADSL packages and has a good news feed.
BT Line Test - Find out what speed BT rate your line to based on estimated physical distance to the exchange.
