I building my links page and adding tools that are of use to people, search engine links are a must but what other tools are available that are of use, below is a list of what I have already. Feel free to post urls to tools that you use on a regular basis.
http://www.archive.org/ - What did x website look like 1 year ago?
http://www.whois.sc/ - Not as good as it was but still useful.
http://network-tools.com/ - Quite a few tools here all wrapped up in one interface.
http://grc.com/default.htm - How safe are you on the net?
http://www.aliveproxy.com/ - Useful if you hang out in irc and need a proxy to defend yourself.
http://validator.w3.org/ - Does your site validate?
http://www.dnsreport.com/ - Just found this one, thanks Bashar.

http://centralops.net

http://scripts.xrenoder.com/
When's your links page going to be ready?

Oh it's just a personal links page for my site .. I have just bought linksplanet.com and was thinking about starting a directory.

http://www.dnsstuff.com

http://www.checkdns.net/quickcheck.aspx

Websitegarage.com

If it's still there...

nope, doesn't look like it's there anymore, what did it do?

www.siteuptime.com
www.whatismyip.org

free registration, and it will keep track up any site's downtime for you. your free acct will be allowed to monitor 3 domains.

http://www.norid.no/domenenavnbaser/domreg.html.
I hope you're not going to just throw links on a page, but you are going to do some tidying up and maybe describe and review every tool. At least at some stage.