Datanyze (https://www.datanyze.com/) has built a nice business doing this. You can learn a lot from understanding what software a company is using on their website. It's especially useful for generating sales leads.
Some time ago Rob Walling, on Startups for the Rest of Us, mentioned that Datanyze is closer to real time than BuiltWith. So real time, in fact, that you can get a list of sites that have just (yesterday?) started a trial of your competitor's product, and approach them during the buying cycle.
