The Swedish language section of WikiPedia.
From its founding to late 2002, the Swedish Wikipedia saw very little activity. This was likely due to the existence of SusningNu, the world's largest Swedish language wiki founded by LarsAronsson. However, in November 2002, Aronsson decided to add a single banner ad to one of the wiki's pages. This upset some of the contributors and they excerised their RightToFork by moving their text to the Swedish Wikipedia. The fork caused no hard feelings, and the two projects continue to be on good terms. See SusningNu for more details.
Since the fork, the Swedish Wikipedia is among the most active non-English Wikipedia projects.