Khan Academy
Khan Academy: Equation of a Parabola From Focus and Directrix
Given the focus and the directrix of a parabola, find its equation. Students receive immediate feedback and have the opportunity to try questions repeatedly, watch a video or receive hints.
Windstream
Ojk's Precalculus Study Page: Parabolas
Good study tutorial about parabolas. There are examples of finding the equation of the parabola or the focal point and directrix that students can examine and attempt.