For example, assume your search ad generated 5,000 impressions in one day, of which 100 visitors have come to your site, and three have converted for a total profit (not revenue!) of $300. In this case, a single visitor for that keyword is worth $3 to your business. Those 5,000 impressions in 24 hours could generate a click-through rate of between 18-36% with a #1 ranking (see the Slingshot SEO study for more on potential click-through rates), which would mean 900-1800 visits per day, at $3 each, or between 1 and 2 million dollars per year. No wonder businesses love search marketing!
In addition, you can dig into the paid side of search and find out what keywords your competitors are bidding on, and then leverage those keywords for your own organic benefit if you're not already doing so. Search Metrics does this as well, but I've found SEMRush to provide a greater range of keywords and they save more historical keyword data than Search Metrics.
In fact, Google Suggest is the best way to find good long tail keywords that people use to find information on Google. Google admits that while Google Autocomplete (another name for the Google Suggest feature) was not designed specifically for marketing, it can deliver meaningful insights and help stay on top of trends. There are also many clever ways to use Google Autocomplete that can help you with SEO.
The Google Keyword Tool has a variety of helpful features meant to streamline the keyword research process. By entering terms or phrases into the search box, the Keyword Tool allows you to build extensive, relevant keyword lists from one simple interface. By default, the tool displays Advertising Competition, Global Monthly Searches, Local Monthly Searches, and Local Search Trends for each keyword. Additional options allow marketers to show the estimated Average Cost per Click, Ad share, and Search Share.
Long Tail Pro is a keyword research tool that helps you quickly find low competition keywords for your website. In particular, the tool is built for people doing SEO (search engine optimization) as opposed to people doing paid ads (Google Adwords PPC). I'm sure there are some advertisers that use the tool for paid ad keyword research, but the tool was built for SEO keyword research.