The most important keyword locations are in the first paragraph of text, page headings, and title tags.

Term Extractor is designed to examine a web site and report on terms notable to a search engine. It lists the results as one word results through five word. It's difficult to tell how representative the results are, but they are fairly consistent. You'll also detect things that appear prominent but which should not be, and perhaps vice versa.

Linkscape offers a rating from 0 - 10 very similar to PageRank, except out to two decimal places. I'm watching to see if it is predictive of PageRank changes, but if nothing else, it's an easy metric to track over time and watch for trends and effects of notable changes.

Trifecta is pretty neat, but there are some hassles. The site requires registration, and only allows free accounts one report per day. The report measures a variety of metrics (visible via the tag, lower on the page -- don't miss it) and then aggregates those findings into a total score. Some tool sites spam you if you register, but no email verification is required at this one, and they are a more prominent firm, presumably above such tactics. Before you run the report, make sure you select "page" or "domain."

Term Target looks at a single web page and key word of your choice, and reports on how well optimized that page is to show up in searches involving that word. It's useful for fine-tuning word arrangement after you have selected the keywords and key phrases you want to compete for.

Google Adwords has a useful keyword tool that may require adding AdWords to any existing Google account you have. It reports popularity of keywords via the number of search uses in the last month, and suggests similar words and phrases. Below is a segment of the report chart, showing results for some searches of interest:

Keyword Tool Tips

There are lots of free keyword counting tools on the web, but they don't all do the same thing. Some count single words, some also or alternately count phrases, some include words in title tags and some don't say what they include. Use keyword tools to home in on your desired word count, trying to keep keywords in the 3 - 7% density range. Then use your computer's FIND to help you do a manual count to verify the final result.

I find that some tools are faster at picking up changes than others. Some appear to start caching your results after a few uses, making it difficult to use them for an extended page modification session. As I use them more, I intend to star the best ones.

More Keyword Tools:

seobench

Keyword Density Checker

SEO Chat

Don't Over-do Optimization

SEO Overkill explains limits and what is too much or too far.