Since http://captain.at has apparently gone awry, I've copied his code here and added some enhancements.
A simple tool for designing active filters using voltage-feedback opamps. All calculations refer to the
values in the Picture (C1, C2, R1, R2, R3)
Type in the center frequency (fc), bandwidth (B), gain (A) and choose a value for the capacitors (C1, C2 = C; 10nF is a good value to start) and click on "Calculate".
Light-grey fields are input fields - white fields are output fields for the calculated values.