Enter keywords into Overture’s Search Term Suggestion Tool, and you’ll learn how many times it (and related phrases) were searched on last month.
Another tool which automates the process of keeping on top of keyword trends is Wordtracker. It’s a paid subscription service, but offers a free trial.
It’s no use peppering your site with keywords, if no one is looking for them in the search engines!
On a related note, here are four search engine pages that provide glimpses into search behavior and patterns: Google Zeitgeist , Lycos 50, Jeeves IQ and Yahoo! Buzz Index.