There are some interesting tools to help users find the appropriate keywords:
Search-based Keyword Tool for Google Adwords
Questo strumento allows you to find a bunch of keywords related to a specific set of keywords, or to some content (available from URL or pasting the text). It shows several statistics for the matching keywords, like the approximate number of search queries, so you can choose the most used ones, and get a better search engine placement.
Textalyser is a powerful text analysis tool. It shows the frequency of words in the text, so you can pick the most used ones as keywords. In order to get a better search engine placement, every keyword should have a 2.5 to 5 percent frequency.
Keyword Analysis shows the density of specific keywords in a text. It works in real time: you enter a keyword and you see the current density. This way you can refine the text and get the desidered density for your keywords.