A number of websites require free registration to view their content, however, for the casual browser getting a pop-up in your face is an annoyance you'd rather do without. Pinterest is one such site and if you're not keen to hand over your details, here's a simple tweak that'll do away with the interruption.
Making searchers everywhere happy, the GreaseMonkey (or TamperMonkey for Chrome users) script is simply called "Pinterest without registration". Some modal popups can be deleted via a browser's developer tools, but others require a little more finesse to restore full functionality to a site.
As gHacks' Martin Brinkmann points out, there are more general solutions available, such as "Remove It Permanently" for Firefox. Sometimes though, a surgical approach is better, in case you want to minimise the risk of other sites being affected, which can happen with haphazardly coded extensions and add-ons.