As I don't have time to make a detailed full comparison. I'll only list the extras that Nimbuzz have and fring do not.

Nimbuzz Extras:

- Nimbuzz Buzz (killer feature)
If you are offline. Your buddy can Buzz you and your phone will ring
- Conference call (killer feature)
I tried a conference call with buddies on Nimbuzz, MSN and Skype
- Voice messaging (killer feature)
It works online/offline and from the Widget
- Web Widgets
Use it with Facebook and other social networking providers
- Chat rooms
Chat rooms with people from all over the World
- Location sharing
Your buddy can see where you are on the Map
- Buddies in the list have their chat provider icon next to them. It's clear what network (Skype, MSN, Yahoo, ICQ, AIM, etc.) they are on
- Aliases
You can give each buddy your own Alias that will replace his Nickname in the buddy list
- Finally, you can send a picture directly from your phone camera. Nimbuzz will launch the camera, you take the picture and then send it

That's all. I hope I did not miss any other feature. You can download Nimbuzz directly from your phone: m.nimbuzz.com

Enjoy!