Composing Mathematics Papers

I was just wondering if anyone had recommendations for programs/document editors that are good for putting together professional-looking formulas and papers? Free and Windows-compatible is always nice, but "free" non-windows apps would work as well. A quick googling led to nothing of use.

Thanks guys.

The equation editer in office 07 was good enough to do my highschool stuff in. Havn't tried it for anything higher though (took physics, chemistry, Maths C (Queensland's highest maths course)).