Mine is John Frieda's Frizz-Ease Secret Weapon Flawless Finishing Creme.
It truly is my secret weapon. If I'm having a bad hair day, this instantly makes it looks at least 5 times better. It keeps my hair shiny and de-frizzes it very nicely. I've only ever used it on my straight hair and wavy-ish hair, so curly girls proceed with caution. And it's only about $6!! Yay!
What's your hair's secret weapon?