How much is a keyword worth to your website? If you own an online shoe store, do you make more sales from visitors searching for "brown shoes" or "black boots"? The keywords visitors type into search engines are often available to webmasters, and keyword research tools allow us to find this information. However, those tools cannot show us directly how valuable it is to receive traffic from those searches. To understand the value of a keyword, we need to understand our own websites, make some hypotheses, test, and repeat—the classic web marketing formula.
After the Panda update rolled out, the latent risk in such a strategy could (typically would) vastly exceed the direct cost of the content, as poor pages on one part of a site could drag down the ranking of other pages on the site which targeted different keywords. Some sites have seen their search traffic fall over 90% with their ad revenues falling even faster. Demand Media went from being worth a couple billion to tens of millions of Dollars. ArticlesBase.com sold on Flippa for $80,000, but was making over $500,000 PER MONTH in profit before getting hit by Panda. Many other Panda-torched sites like Suite101 have simply went offline.
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 a nutshell, Long Tail Pro helps you quickly find keywords in bulk based on a seed keyword that you input. In addition to returning hundreds of related keywords, the tool also shows search volume (monthly search volume), advertiser bid, number of words, rank value, and my favorite Keyword Competitiveness (helps you judge the competition and keyword difficulty).