A common requirement in competitive intelligence is to compare how competing brands are trending. There are free online tools that enable these comparisons, not always in depth but certainly they are good enough for sanity checks.
The first is Google Trends. This tool compares the search frequency of given search terms. So, for example, how often people search for Volkswagen and Nissan. The data can be broken out a little, for example by region and time period.
A second tool is Compete, which provides data on visitor traffic to websites. Many companies have better, paid-for equivalents, such as Nielsen, but where that data is not available, Compete is a quick, free tool. Like the original such tool, Alexa, this data is more indicative than to be relied upon, but it is still a useful guide.
Perhaps less relevant to a competitive intelligence search, but fascinating nonetheless, is the new Google Ngrams. Here Google analyses its dataset of scanned books dating back to forever.