Skype App
I don't know what I would do without skype on my laptop and on my iphone. Not only is it the best instant messaging tool for typing messages but with VOIP and the ability to make voice and video calls, it allows me to be more productive at work and stay in touch with family and friends globally, for FREE, any time, anywhere.
The app could be improved by allowing skype text and video conversations I have had on other devices (laptop, desktop) to be synched up with my mobile so I can revisit them when ever I want. This could be done via the history functionality for example.
Another productivity enhancement would be to allow collaboration and sharing of content from one mobile to another, albeit at a little more basic level (smaller images, shorter videos, focussing on a part of a design rather than a larger whole) as screen real estate is more limited till projection phones become the norm!