Webmaster Tools: Keyword Density Analyzer Web Master Tool

Using a keyword density analysis tool can make it easier to find a few good keyword phrases to use.
Search engines may place significant weight on domain age, site authority, link anchor text and usage data.
The page title is typically weighted more than most any other text on the page.
The meta keywords tags, comments tags, and other somewhat hidden inputs may be given less weight than page copy.
Page copy which is bolded, linked, or in a heading tag is likely given greater weighting than normal text.
Weights are relative. If your whole page is in an H1 tag that looks shady, and it does not place more weight on any of the text since all the page copy is in it.
You probably want to avoid doing things like bolding H1 text as it is doubtful it will make a page seem any more relevant.
Lots are queries are a bit random in nature. When people tweak up page copy for an arbitrarily higher keyword density they typically end up removing some of the modifier terms that were helping the page appear relevant for many 3 and 4 word search queries.
Latent semantic indexing, or other algorithms with similar intent, may look at supporting vocabulary when determining the relevancy of a page. If you pulled the keyword phrase you were targeting out of your page copy would it still be easy for a search engine to mathematically model what that phrase was and what your page is about given the supporting text?
When people focus too much on keyword density they tend to write content which people would not be interested in reading or linking at.