How popular is your site?

According to Shari Thurow's Book Search Engine Visibility. The key to ranking well on search engines like google is to have a site that is easy to navigate, full of keyword rich copy, and a site that is popular. Well the first two are easy to test, but the popularity aspect can be a little bit harder. So here are tools to help

Link backs
If people are linking back to your site then they must like you. Also the more people that link to your site the more relevant your content is for search.
If you type link:www.YourSite.com into google you will see all the sites that link to yours

Popuri.us
This site gives you your page ranking on various search engine sites.

Update (4/12/2007)
Big thanks to My web marking professor Steve Eisenberg for recommending This Link Popularity checker. It allows you to compare your site to 3 of your competitors.