Google Trends is a new tool from Google (well…duh) that will graph different popular search trends over the past few years (or isolated, if you so choose).
I see this tool as being an asset to any webmaster. For one thing, it can show you how to best guide your clients’ keywords for SEO. I suggest this because odds are you won’t be an expert in what’s popular in every industry which you have a chance of working with.
Secondly, since you can isolate to your area (sometimes even your city, depending on the number of searches available) it can show pretty well how you can gear your SEO specifically toward what people in your area are searching. For example, comparing the search trends for web design, web sites, and web masters could show you where the emphasis is put in your key demographic (your area) and what direction you could gear your site.
One use of Google Trends below: comparing worldwide searches for web design versus WordPress. Pretty amazing.
(Hint: WordPress is in blue.)