Moz.com (formerly SEOMoz) just released a list of free SEO tools: The 100 Best Free SEO Tools and Resources for Every Challenge by Cyrus-Shepard. There are items on this list that I didn’t even know about. (Check out BuiltWith. I was fascinated for nearly 30 minutes plugging in client sites.) I highly recommend bookmarking this article for future use.
What do you think of the list? Are there any others you would add?