http://www.live-keyword-analysis.com/ I don't know if you have all seen this before but I was impressed. You can paste your copy into this tool - enter the keywords / phrases and it gives you the density (so what)... the good bit is you can edit the copy then and there in the tool and the density measure will change live as you edit the copy.
But the question is what is the value of keyword density? Has anybody got any proof that it effects SERP placement? I personally think it is a non starter. Just write naturally. Your keywords will be included in the natural flow of the content. Keyword density like PR obsessions and exact time of the Google's cache are all for the anally rentive SEO practitioners. Thy all have little, or nothing, to do with getting your site placed well in the SERPs.
The issue with keyword density for me is when I started writing online it was one of the very few pieces of information I had about writing webpages. This developed a bad habit (I over used keywords). I'm more likely to need to decrease the number of keywords than increase them so thats why I find something like that useful. I don't pay any real attention to them now other than grouping so the description turns out nice next to my serps on key searches and checking to see I keep it under 10% density. The problem with some of my pages is I tend to use my keywords in ancor text in my menu for the anchor benefit and thus want to avoid using them again too much in the content as I now already have a number of uses in my menu. I agree density is no real concern unless I am a little more dense than I should be .