I'm looking for recommendations for C++ libraries for non-linear programming (*not just for integers!*).

Preference is for actively maintained F/OSS solutions, but actively maintained commercial solutions offering free evaluation versions are acceptable.

Thanks in advance!

**Edit:** As requested, here are more details:

The application I'm working on is simple, and it's about minimizing polynomials of degree 4 and higher (up to 11) with a small set (~20) of constraints expressed as inequalities (again, as polynomials, degree <= 4). I might also, occasionally, have to throw in some trigonometric function, though. In any case, it's always continuous, differentiable functions I'm dealing with. The number of variable ranges from 1 to 12, for now, and won't go much beyond that.

Finally, I need a solution that works on Windows, but cross-platform ones would be preferable.