I recently came across this nifty google tool to find what are the most popular search terms. It dynamically show information on most searched terms in a period of time. It’s called Google insights for search.
Using that tool, You can get a detailed description on the increase or decrease in popularity of a certain term.

As an example, the amount of people who was looking for “”web design” has decreased over the years.. It’s going down down and down…

Now check the term Responsive web design.. It’s going Up, Up and Up

It shows us that responsive designs are more in demand and desktop only web sites are no longer appealing.
Finding the popular search tearms are  very helpful when doing SEO for a page or a web site. It also give you a clue on what you should be doing, if you need to do well in the present and the future.