As for The Register, their top application choice ended up being Skype:
But the benefits are undeniable: if you have a Skype-in number your mobile will ring when someone calls it, and you get access to voicemail, instant messaging, and your Skype contact list from wherever you are. The ability to make calls for pennies from anywhere with a Wi-Fi connection is a definite plus for anyone who frequently finds themselves out of range of a decent GSM signal, or out of the country.
And the other nine? Why not head over to El Reg and find out.