Torchwood is a Doctor Who spin-off, as is The Sarah Jane Adventures. While the latter is specifically for children, and Doctor Who is sort of in the middle, Torchwood is very adult. Actually, I mistyped; I've seen all of Torchwood except the latest season.

The main character on TW is Captain Jack Harkness who has appeared in several DW episodes, and does, I believe, count as a Companion, since he has traveled in the TARDIS (more than once).