The two biggest and most comprehensive fansites for Cities XL are Simtropolis and CitiesXS. Simtropolis is a SimCity 4 fansite with the largest English forum for Cities XL. CitiesXS is a German fansite with an English section devoted entirely to Cities XL; it has the only other Wiki for CitiesXL and is the most comprehensive one--and the only one in German (and English). This wiki uses some information from CitiesXS with their permission. The vast majority of custom content for Cities XL comes from these two sites with CitiesXS contributing twice as much custom content as Simtropolis. Génération City is a French language fansite originally devoted to City Life and XL Nation is new.
CitiesXS Main Site (German but navigable to English) 
Cities XL section of Simtropolis 
Génération City 
XL Nation