What do you want to do? If you want to calculate photonic band diagrams, there is a free package offered by MIT that is likely the most accurate and powerful tool available. It may take you a little work to get running, however. Just go to Google and search for MIT and "photonic crystals" and it will take you there. They have a very nice website and introduction to materials.
If you want to model scattering from a photonic crystal, you may want to look at the free Translight tool. It is based on the Transfer Matrix Method. There are also plenty of good and free FDTD codes available on the internet and in various EDABoard discussions.
-Tip