Crossovers
As you all should know, almost all of the characters in Tsubasa Reservoir Chronicle came from other CLAMP manga. These characters have been correctly dubbed "crossovers". As of now, I have neither the time nor patience to compile a list of all the crossovers in TRC, yet these two links will bring you to some wonderful lists.
THE ALMIGHTY TSUBASA CROSS OVER LIST!
Toll Road: the CLAMP crossover project