You can find many useful web based tools in this page [soon]. For command line tools, you can check here. If you have any tool which you would like to recommend other security professionals please leave a comment.
SSL Certificate Config Checker
This tool from Ivan Ristik at SSLLabs allow you to check your Web Server’s SSL Certificate for the following
- Proper Certificate Chaining
- Strong Ciipher Support
- PCI Compliance and more
This website has tools for e-mail, dns, and web tools. I like their open relay tool under the e-mail section.
What is My IP
You can use this website to learn your external IP if you are behind a NAT router. More importantly you can search IPs and learn the host names, locations and if the IPs are proxy servers’ IPs. Moreover, you can check IPs if they are blacklisted.
AVG Online Web Page Scanner
The AVG LinkScanner® Drop Zone lets you check the safety of individual web pages you are about to visit. You may have received a suspicious link in an e-mail — just copy the URL (web page address) and drop it into the box below. LinkScanner® will examine the web page in real time to see whether it’s hiding any suspicious downloads. Even innocent web sites can contain hidden “drive-by downloads”. Use the AVG LinkScanner® Drop Zone to inspect:
- Links forwarded by friends
- URLs displayed by search results
- Links containing odd characters
- Any site you’ve never visited before
McAfee® provides an online tool that enables you to check if a site is categorized within various versions of the SmartFilter Internet Database or the Webwasher URL Filter Database. After you check a URL, this tool also allows you to suggest an alternative categorization for a site.