Ranking Checker by SEOmoz
For no cost, you can check your rankings for up to five keywords per day, and it keeps an excellent little log for you of how your keywords rank day after day — a log that you can download as a CSV file if you need. A dedicated SEOer can check about 150 keywords per month with this tool, provided you can deal with your updates being that rare.

MultiRankChecker by iWebtool.com
Sometimes, you just dont’ want to manually check the PR and Alexa rankings for your client and their seven major competitors all at once. Enter MultiRankChecker and you’ll get results in seconds.

Fiddler by Fiddlertool.com
Fiddler requires the .NET framework for Windows, so not everyone will be able to use it — but for those who can, it’s grand. Fiddler allows you to analyze all of the background communication that happens between a browser and the servers. It’s excellent for debugging a webpage that isn’t doing what you want it to in all of the browsers you want to do it in.

SEO Analyzer by Sitening.com
SEO Analyzer gives you a report that shows a detailed list of SEO components and how the assessed website stacks up to them. It’s not any substitute for a professional SEO assessment, but it’s a great tool for making sure that your website has the basics covered before you turn it over to the experts.

Backlink Checker by WeBuildPages.com
This is probably the best free backlink checker on the Web today, it’s simply a useful way to keep track of your backlinks, or, more importantly, analyze a competitor’s backlinks and see where they’re getting their juice from.

Are there more? Of course — probably hundreds of great free tools — but these are the ones that our SEO experts use on a daily basis. Try them out and see for yourself!