Actually there is a package that has some free use called IBP - Internet Business Promoter and it analyzes your page and the top 10 pages under a specific keyword and compares the difference. Its a cool tool. The premise is that if the top ten are a model of good optimized sites it will give you feed back to make yours comparable to the top ten.
I'm with JamesR and Oaf357 about SEO software. For all sorts of reasons, I wouldn't rely on software to do my thinking for me.
Software can be helpful once you know what you're doing. I think the Google Toolbar is a great tool... lets you look at text distribution in a page, check Google backlinks, PageRank, etc. I also like NoteTab Pro for its Text Statistics tool (it's an OK html editor too).
There are a lot of online tools for looking at keyword density. An excellent set of online tools, including a density analyzer, is on WebmasterWorld's sister site, at:
The engines themselves are great tools... both Google advanced search and AllTheWeb have some excellent features that are helpful in checking out searches and sites. Be careful of third party tools that send automated queries to these engines, though. Google specifically prohibits automated queries in its terms of service: