Have you ever noticed how Etsy feels like a combination of Pinterest and eBay? There are a lot of similarities between websites today; for example, if you added Twitter and YouTube, you would come up with a service very similar to Vine. Even Tinder, one of the hottest online dating services, is basically like a Snapchat version of OkCupid.
Subjection will also lead you to new services. Take Google Drive, for instance, which is pretty much like the best of Google and Microsoft Office together sans any real Microsoft influence. Or how Imgur is basically like Reddit minus Reddit.