Google trends
Google trends is a great tool that allows you to type in any word or phrase and see how popular it has been in google searches - for any year, any month, and different countries. google.com/trends
Keyword tool This tool from google allows you to type in your keywords and find all related keywords, how much advertiser compeition there is for those keywords, the search volume for those keywords and other information. adwords.google.com/select/KeywordToolExternal
Search Engine ranking
Check if your site rates in the top 1000 for your keywords in google, msn, yahoo and ask.com (Hint: Do not expect this to happen straightaway!) http://www.googlerankings.com/index.php