Let's keep this an ongoing thread with good SEO tools to help the budding and advanced SEOer.
Show Pagerank - Shows pagerank over different Google datacenters.
Unique backlink checker - Displays domain backlinks. I find it very useful as it doesn't show all the (unnecessary) individual pages that are indexed as a backlink.
Google/Yahoo Rank Checker - Sometimes is off by 1 or 2 positions, but is fairly accurate.
Broken Link Checker - Search engines like it if your site isn't filled with broken links.
McDar's Keyword Analysis - Displays the PR/backlinks/pages of the top 10 SERP listings.
PRSearch.net - Great site with tools for checking rankings, and link pop (without img verification).