Interest in encrypted communications has risen in the wake of whistle-blower Edward Snowden revealing the extent to which the NSA eavesdrops on foreign communications.
For free encrypted voice communications, RedPhone will detect whenever someone calling you is also using the app -- a requirement for use -- and give you the option to make the call secure. The software was originally designed by Whisper Systems, which was co-founded by the security researcher known as Moxie Marlinspike, before being bought in 2011 by Twitter.
Although not free, another popular app is Silent Circle, which offers a service for encrypted communications between smartphones, as well as encrypted text messaging, encrypted VoIP audio and video calls, and encrypted email. Beyond encrypting communications, the service also can disguise the identity of users. The developer says the app has been independently audited to ensure that the underlying code is free from back doors.
Current Silent Circle app users should upgrade immediately, however, as security researcher Mark Dowd of Azimuth Security recently identified vulnerabilities in the ZRTPCPP protocol. Originally built by Phil Zimmermann, the creator of PGP, the protocol is used to secure Silent Phone communications. Those bugs have now been fixed, and developers at Silent Circle -- and a number of other projects that use the protocol -- have pushed fixes.