How to search for related keywords?

I often use a web site to search for any related keywords, the trouble is that most of the words that it comes up with have no meaning at all, and are totally irrelevant. Are there any other good tools that are available to the same purpose?

And also, what’s the best density for related keywords? I used the kdindex of googlerankings.com, but am still unsure about how these are calculated? Would someone please clarify more about this? I really would appreciate your opinions and advice. They will be very useful to my entire SEO process.