You come across things that you end up using all the time. This is just a short list of some of the tools i'll use daily in my job as a Linux admin.
PasteDB.com : Pastebin clone : http://www.pastedb.com When working with code, bash scripts, etc.. you'll eventually need to share some with another human. The easiest way to do this is to find a good pastebin type site.
StrongerPassword.com : Strong password generator : http://www.strongerpassword.com When setting up users, you find yourself gazing around the room trying to think of a good password to hand out. Make it easy by using a website that can generate strong passwords on the fly. Add capital letters, integers, symbols..
WhatsIP.net : What is my ip address : http://whatsip.net When speaking to someone who is possibly blocked by a firewall, you'll need to know their IP address. This is a nice site that you can send a client to. Easy to remember, short and not overpacked with ads.
Check-Pagerank.org : Check Google Pagerank : http://www.check-pagerank.org This isn't really a 'Linux admin' tool, but if you're into Linux, you're likely a lot of things.. webmaster included. This site will help you to keep an eye on your website(s) pageranks.