The name which you have used to start the thread is itself the name of the tool for which you searching for. Really, Keyword density checker tool is the very best tool for checking the keyword density.
Is it really the keyword density tool you want to have or is it the keyword analysis tool? Keyword density tool is only used to check the denseness of keywords in an article, so if you only have 2-3 words to check, then I doubt you're asking for the right tool. If what you want is the tool to determine the competitiveness of each keyword, then I suggest that you use Google Adwords Keyword tool. It's a free keyword tool that can analyze the trend, level of competency and # of clicks per month.