This is the best application i have installed in my mobile phones: iphone and nokia n95.
Using fring you can online all you messengers from YM, ICQ, AOL, Google Talk, MSN, SIP, Skype and Fring. If you have WIFI in your home, office, perhaps you’re in coffee shop or any access point, this application is incredibly useful which makes you always get connected. The best thing is you can call anyone for FREE. How cool is that!!
It’s currently in beta version but all functionalities you expect are working well. http://www.fring.com/