Well, for the Netherlands....
Feyenoord and Excelsior are friendly to each other. Excelsior being like the "satellite club" to Feyenoord.
Twente and Heracles share youth and scouting together. They're seperated by like 20 km's and because Twente always played on a higher level there is no real rivalry.
For a few years people in Limburg (south of the country) have been talking about an FC Limburg. This would have to comprise of Roda, Fortuna Sittard, MVV and VVV. Wouldn't say those 4 are real friendship clubs right now, though.
This thread inspired me to a much more interesting one, though.