According to SEOmoz each year, Google changes its search algorithm up to 500 – 600 times. While most of these changes are minor, every few months Google rolls out a “major” algorithmic update that affects search results in significant ways.
One of the most useful ways to use the list is to compare your site’s traffic with the specific dates outlined for major algorithm updates. If you notice a drop, it’s wise to dig into the algorithm change and see what you can do to see why, specifically, your site suffered.
Also an Infographic showing up-to-date change history of Google Algorithm-