You can obtain the coefficients of Hilbert class polynomials. This application is assumed to be called from a program, via the URL:
http://hilbert-class-polynomial.appspot.com/disc
where disc is a discriminant of imaginary quadratic field. The result will be a list in JSON, representing the coefficients in ascending order. If the polynomial has not been registered (or disc does not represents a proper discriminant), the result will be null.