Android: Getting a call from an unknown number forces you into making a quick decision based on no information: do you want to talk to this person or not? Contactive helps solve this by putting a name to the number.
This app isn't the first to try to demystify unknown numbers, but the service uses a crowdsourced model for identifying business numbers, so the list is constantly growing. In addition, incoming calls from your contact list will display social media updates from a wide variety of sources.
While some manufacturer variations of Android will do this, it's a handy feature that others (including Google's own stock dialler) do not have.