If you aren't impressed by the feature set of Android's default phone calls, Ultimate Call Screen HD will add a load of new features to your calls, including full-screen contact phoots, a call blocker, and the ability to reject a call by turning the phone face-down.
The features of this app are almost too numerous to list, but if you use your phone to make phone calls (you know, that thing it was originally intended for), it's a must-have. Not only does it let you assign full-screen pictures to your contacts (something Android should have done since the beginning), but you can also put together an advanced blacklist for blocking certain callers, as well as use a number of sensor-based actions that let you reject calls just by turning your phone face-down. If you buy the $US3.99 pro version, you can also add different themes, set calls to mute or speakerphone using sensor-based actions, and more. Check out the link below to see the full feature list.
Ultimate Call Screen HD is a free download for Android devices, with a $3.85 Pro license available.