Here are a few websites that I have been using or have been hearing good reports on.
http://www.seohosting.com/tools/googlebannedchecker a good site with tools..you can check to see if you were banned by Google.
http://www.seochat.com/ very good site with lots of SEO info...and lots of SEO Tools. You will need to read a lot which is good for those serious about SEO
http://www.mattcutts.com/blog/ Matt is one of Google's engineers...he shares a lot of helpful tips on websites. If your site gets posted there...it's basically an honor!
http://www.iwebtool.com/tools/ VERY NICE site with nice SEO tools...user friendly
http://pr.blogflux.com/ site has been down the last few days..but good source to check out your Google PageRank.
http://www.domaintools.com/ pretty good site...I like the WHOIS feature which shows a SEO SCORE....I havent seen any site that does that.
http://www.seoworkers.com/tools/ site to analyze your meta tags
http://www.websitegrader.com/ gives an overall score for your website. Keeps you on track. VERY NICE.
http://www.submitexpress.com you can go here to check link popularity, meta tags, and to submit your url to various websites.
http://www.warriorforum.com probably the largest SEO forum that I have seen! Lots of info on SEO, PPC, ADWORDS, EBOOKS, etc
http://www.sitereportcard.com/ just found this one....nice site that finds errors in HTML etc

Robert, great list that's most appreciated. It's bookmarked and know of a few of your places already...
cheers
Thanks for these helpful links! I've checked one out already - the first one - and it appears to offer some terrific checking tools. I've bookmarked for reference.
Thanks again!
another good site i found....
http://www.sitereportcard.com/
Great post!
Very good info...thanks for sharing it with us!
Take care & good luck!