Many people are already familiar with Moxie Marlinspike's WhisperSystems because of their Android apps: RedPhone for encrypted calls, and TextSecure for SMS messages.

The release of Signal is a pretty big deal for iOS users; previously, we had to consider a paid option like Silent Circle, or a larger corporate option like FaceTime Audio (which isn't really the same).

Although I haven't actually used the app yet (it's registering now), the screenshots appear to be a fairly direct port of RedPhone to iOS.

Edit: Yep! And it looks like Signal users can make secure calls to contacts with RedPhone installed, too. Very nice.