Indeed.com has a tool similar to Google Trends called Job Trends. Basically, Indeed.com goes through its job postings and figures out how frequently a word / phrase appears. You can put in a single word or multiple words, and the tool graphs “% matching job postings” along a time scale starting from February 2005.
There are many ways this tool can be used. For example, what’s the popularity of a particular tool? What’s the relative market share for several competitors in a space? For the second question, I entered several key business intelligence tool vendor names, and the result is shown here.