There are eight followers that you can get at the indicated locations with the appropriate karma.

Butch: At Rivet City after completing the "Trouble On The Homefront" quest. Requires neutral karma.
Sergeant RL-3: Purchase this robot from Tinker, a salesman wandering the Wasteland. Requires neutral karma.
Fawkes: Save in Vault 87. Requires good karma.
Star Paladin Cross: Found in the Citadel. Requires good karma.
Jericho: Found in Megaton. Requires evil karma.
Clover: Buy this slave in Paradise Falls. Requires Evil karma.
Charon: Purchase the contract for this Ghoul from Ahzrukhal in The Underworld. Requires any karma.
Dogmeat: Find this dog in the Scrap Yard. Requires any karma.