Here are some Internet sites with useful information about Trondheim, in English:

The official Trondheim internetsite
Official Trondheim tourist guide
The history of Trondheim
Virtual Trondheim
Photos from Trondheim
A small walk through Trondheim
Weather-forecast
VisitNorway.com
Norway.org
Lonely Planet: Norway
Travelling in Norway

And here are some more Rosenborg-stuff, in English - of course:

Kjernen - Supporters' Club
An introduction to Rosenborg

...but do you really need any other RBK website than Rosenborg Web?

Norwegian football in general (news):

UEFA.com: Norway
PlanetFootball/Sky Sports: Scandinavian football