When an unknown number calls, you can check the Should I Answer? evaluation to get an idea of the number's credentials as given by the user community, as well as see info on the general nature of the call (spam, scam, etc.).
Other interesting features in Should I Answer? include the ability to automatically block calls or create your own blacklist. You can block calls from hidden numbers, foreign numbers, etc. And you don't have to worry, because nothing you do will block any of the numbers in your address book.
Should I Answer? is a very interesting app. With it, you can check your device, see the number that's calling, and easily decide if you want to answer. The best part is that the app doesn't need the Internet in order to work.
Requires Android 2.3.2 or higher