United Nations Intelligence Task Force (UNIT)
Formed to protect the world from dangerous alien threats and alien invasion, UNIT is not only an ongoing ally of the Doctor and his companions, it’s also where he was employed for a time. Although he was an agent for good, the Doctor had been regarded by his fellow Time Lords as something of loose cannon. Consequently, they prevented his TARDIS from being capable of leaving the Earth and effectively stranded him on that planet. In order to work on repairing what the Time Lords had done to the TARDIS, the Doctor signed on to serve as a scientific consultant for UNIT.
Of course in the early days of the show, UNIT were pretty much a low key outfit, although there was the implication that they were bigger than just the UK side of things. With the reboot of the series though, UNIT has become far more high tech and had a definitely higher profile and far more influence. But with great influence and better toys has come less appearances.