Developer: Skype (now part of Microsoft)
Cost: Free for basic use; if you want to make Skype calls to non-Skype users, pricing starts as low as $0.012 per minute.
Available on iOS too? Yes
Skype is a must for business users who travel overseas often, or for workers who must often collaborate with people overseas. The app will save you big time on your telecom bills, giving you the ability to make free calls, no matter where in the world you are, to other Skype users. The mobile app also lets you make video calls from your smartphone, share your screen, and send and receive instant messages.