I try a few of these tools and sites out every now and again here are a few for you to try yourselves and see which ones you like:

The Free Dictionary

I consider this to be one of the most valuable resources out there for keyword optimization. In addition to just showing you the meaning of a word, the dictionary also shows words of the same root and thesaurus entries. Use it to approach your keywords from more than one angle.

Google Trends
Google tends to make software that is easy to use, extremely helpful and best of all, completely free! Their Google Trends service is no exception. It analyzes Google search data and shows you how popular that search is over a period of time. and while they don’t claim to be 100% accurate, having a general idea of the popularity of your search terms will help you whittle down your SEO terms.

Google Blog
Add this blog to your regular blog roll now and thank me later!. These masters of SEO clearly know a thing or two about search. Learn tips, tricks and go behind the scenes with the geniuses behind the service that has taken the world by storm.

Keyword Discovery
This service is on the expensive side, but has some decently priced plans. If you need help with optimizing meta tags, beating out the competition or maximizing your PPC campaigns, Keyword Discovery can help.

Keyword Research FlowChart
Ever wonder how to determine if your SEO efforts will pay off. Use this flow chart to help navigate the SEO waters with easy to follow instructions each step of the way. Larger version and downloadable PDF available on the site.

Keyword Typo Generator
Enter your keyword of choice and then enter the type of error you want to search. Wrong key? Missed Characters? Double Characters? This keyword Typo Generator has them all covered.

Meta Glossary
Stop searching for your dusty physical copy of the Dictionary and start using MetaGlossary. They scour the internet to provide you with current definitions for any word you can imagine.

Ranks
From a company with 10 years of solid SEO experience, comes a great keyword density tool. Check to see what keywords your competitors are using and use the results to boost your business. This is a great resource!

SEO Workers Analysis Tool
Don’t know the ranking potential of your web page? This tool is a great one because it provides you with keywords, all urls located in the page, keywords found in anchor tags and more! And it analyzes not only meta tags but the same places internet spiders check as well to ensure all bases are covered.

SEO Tool Set
Multiple useful tools in one place make this site a go-to for anything SEM. From a SEM tool bar, a server checker (that lets you know if something in your server is erroneous) and competition research, you’ll find yourself using the resources on this site regularly.

Similpedia
As its name suggests, this site helps you find similar content on…you guessed it – Wikpedia. Just enter a URL or paragraph of 100 words and you’re given all of the wikipedia articles that are related to that content.

Tip of My Tongue
What was that word? It starts with Q and has an I in it…and an R, I think… No more guessing! Tip of My Tongue  Enter partial words, letters or meanings to get that word off the tip of your tongue and into your keyword cache.

Traffic Travis
Besides the fact that Traffic Travis was highly rated by CNET (4.5 stars), this program combines a bunch of useful tools: keyword and competition searches, your site’s ranking amongst the top four search engines, and a point-and-click advertisement tracker. Add all this to the fact that the layout is simple and the program is easy to use and Traffic Travis is an online marketer’s dream come true.

WordTracker
Has a free keyword suggestion tool. Known as the Grandfather of SEO sites, but they also have linkbuilder and strategizer tools. Perfect if you need continuous (versus sporadic) access, high level details about a niche, and a few projects at a time.

Which worked best for you? And would you add any more to the list?