Mixing pairs: ultramarine blue and burnt sienna

Studio Products ultramarine blue, Doak burnt sienna, Rembrandt titanium white.

I mix these two often. They are not exact complements, but they form a useful set of neutrals. It is particularly helpful that you can get a very dark neutral from them, which I often find to be a good substitute for black.

Update

21 June 2008: For lighter neutrals (grays) I tend to use ultramarine and raw sienna, which makes a more pure neutral. But for darks, burnt sienna and ultramarine is the way to go.