Over the course of the series a number of TARDISes have been seen besides the Doctor's. The first was the Monk's TARDIS in "The Time Meddler". Later, TARDISes belonging to the Master ("Logopolis", et al) and the Rani ("The Mark of the Rani", et al) were seen.
The Modern Era began with the revelation that, as far as he was aware, the Doctor was the last surviving Time Lord and his TARDIS the last TARDIS, a fact the TARDIS herself indicates in "The Doctor's Wife". In that same episode, the Doctor and "Sexy" (a name she appropriated) cobbled together a makeshift TARDIS.
In "The Name of the Doctor", another TARDIS is glimpsed. The First Doctor and Susan (in an event predating "An Unearthly Child") is seen to almost fly away in it until a Gallifreyan echo of Clara Oswald recommended another TARDIS instead.
The next appearance of a TARDIS other than the Doctor's occurred in "Hell Bent" when the Doctor - now returned to Gallifrey, which is no longer trapped in a pocket universe after the events of "The Day of the Doctor" - stole another TARDIS, which was subsequently appropriated by Clara Oswald and her new companion, Ashildr (a.k.a. "Me").
Additional TARDISes have appeared in novels, comics and audio dramas.