Assuming you're using Wordperss, the best free tool for this is Wordpress SEO by Yoast. Under the content area of the Edit page, there's a section for Focus Keyword.

It not only does that, but it covers everything else for your on page SEO including titles for all parts of wordpress, sitemaps, breadcrumbs, etc.

There are other paid options which are good. Clickbumb SEO is recommended by many. I used SEOpressor, I would not recommend it if you plan to have more than one site in your hosting account. It is a terrible performance hog.

I'm using blogger actually...