Fring for iPhone
The latest killer App for the iPhone is Fring. We’ve set this up to work with our studio VOIP system so now when logged onto any WIFI network our iPhone becomes part of the voice network. Genius.
The only downside is that it won’t work over 3G (due I am sure more to O2′s port blocking than Frings technology) and the fact that you always need the app open to receive calls (or how else would O2 make any money) its still a useful tool for free access to an office network though !