Or you can just get the SDK: look on a Photoshop 6 or earlier CD. (Yes, it does work with CS.)
If you’re also looking for a suitable development system: I use, and highly recommend, Apple’s freely downloadable MPW <
http://developer.apple.com/tools/mpw-tools/> for Mac plugins, and mingw <
http://www.mingw.org/> for Windows (which is GNU-based – like Xcode). Complete example projects (and many free plugins) can be found on my web site <
http://www.telegraphics.com.au/sw/>. If you have a decent budget for programming tools, CodeWarrior <
http://www.metrowerks.com/MW/Develop/CodeWarrior.htm> is a nice IDE although not entirely superior to these other options.