My favorites include: Joico K-pak, It's a 10 deep conditioner, Kerastase Nutritive Masqueintense nourishing treatment. All of these are professional products that range from $20 to $60.
Deep conditioners are also called a hair mask.
Shampoo your hair as normal. Apply the deep conditioner and wrap your hair - use a shower cap or a thin towel (baby towels are my favorite when wrapping my hair) and whip out your hair dryer. Apply medium heat for about 15 minutes. The heat will allow your cuticle to open up and absorb all of the yummy goodness that is in your deep conditioner - making the most of it!
Let your hair cool down for a few minutes before rinsing, then rise with cool water - closing the cuticle.
This is a kindness you can do for your hair, but necessary for all of you who have dry, damaged, highlighted or color treated hair.
Repeat every two weeks or so.