3G Unrestrictor allows you to go around the iPhone's limitations and restrictions like not being able to use Skype or SlingPlayer while on a mobile internet connection like 3G or EDGE as well as many other things.
It allows those things by tricking applications into believing they are on WiFi, even though they are on 3G or EDGE.
It allows you for example while on 3G/EDGE/GRPS:
Developers were forced to put those restrictions in by Apple, but 3G Unrestrictor makes those apps "think" they are on a WiFi connection so that you can do all those things.

It works on all iPhone models: The iPhone 2G, the iPhone 3G and the iPhone 3GS
You need firmware 3.0 or higher to be able to install 3G Unrestrictor.
If you use this to bypass something your provider or anyone else forbids and you get caught, you cannot hold me responsible for that.
This is just a warning to protect myself.
3G Unrestrictor has been available for quite some time and so far nobody had any problems with their provider.