G*Power proves both surprisingly easy to use and also surprisingly powerful time and again for me. I use that to perform power calculations for pretty much all of our animal usage applications (we do a lot of hippocampal-dependent behavioral tests while treating the animals in a wide variety of manners), since it can handle linear models without any fuss (it also has a number of quite handy tutorials).

BTW, be careful with post-hoc power analyses. I consider the wonderfully written paper by Hoenig & Heisey to be required reading, if you've not done so.