There is now a way to browse through popular Collections:
That's new. Finding interesting Collections in the past was a bit harder. Someone would have to share one with you somehow, or you'd have to stumble across one on someone's profile, or through the occasional footer cross-promotion we do on some pages. Much nicer to have a decided place for it.
These are "Popular" Collections in the same way Popular Pens works. It's determined by an algorithm based on views, hearts, age, and other things. Whereas the page for Recent & Good Blog Posts are chosen just like Picked Pens. We'll be revisiting all this soon-ish, so that you'll be able to browse all the content in all the different ways, but for now it's nice to have this dedicated home for Collections.