One of the interesting companies which presents its offer at the Symbian Smartphone Show was definitely Truphone. They provide a service that routes your mobile calls via the Internet, giving you free mobile calls to other Truphone users and very cheap calls to anyone else.
Their service is especially interesting when used in roaming, where Truphone allows you to save big. Speaking of savings, until Dec 31st 2006, US users can call across the US & Canada for free, whereas UK users can call free to landlines in 40 countries worldwide.