SEO Book have a free keyword density checking tool but like it says on the site, don't become too preoccupied with it as it's not an important ranking factor.

"google keyword tool" is the best my friend .. all other = 0

I use SEO book for mine. I also use a plugin that tells me what my keyword density is on each page of my site.

Seobook's toolbar has one keyword density checker but I never used it for any of my projects. It isn't something you should pay much attention to.

i think free market samurai is the best for research keyword, or you can try overture.com guyz

Try SEO Quake

i usually use google keywords tool . also recommend traffic travis(someone told me this)