If you would like to optimize your web site through keywords, it’s important for you to remember that they need to be both in the article as well as in the tags. You cannot abuse the keywords, because you could be penalized for that. The keywords you use must have a relation with the article.
Here are some tools for keywords:

https://adwords.google.es/select/KeywordToolExternal
http://www.miva.com/es/content/advertiser/landing1.asp
http://www.digitalpoint.com/tools/suggestion/
http://www.wordtracker.com/

Many will go for the most used keywords available, making it more difficult to be on the front pages of the search engine. That’s why I always use the not so popular keywords.

Source: Chica SEO