What you'll need:
The Adobe Photoshop SDK. To obtain this SDK, you'll need to visit the Adobe SDK / Developer site and get permission to download the SDK. Unlike their other SDKs, you'll need permission to download this one. But all you have to do is ask, and you do not need to pay anything in my experience. Some other sources said that now Adobe is charging people for the SDK blah blah blah but I did not find that was the case.
Software: In addition to Photoshop, you'll need Xcode (Mac) and Visual Studio (Windows). The example projects are for these programs. (If you only want to make PC-compatible plugins then you won't need Xcode.)
Programming knowledge: You'll need to know C++ to work with these projects. On the Mac side, you'll need to create your own GUI using Carbon (see this thread on making a Carbon interface for Mac) or Cocoa. Unfortunately, you'll have to redo the GUI in the example plug-in unless you have Resourcerer to edit the (outdated) resource files / GUI. Cocoa is Objective C and will be necessary in the future for 64-bit plugins (but do a search on the SDK forum to read about any potential issues, e.g. bugs).
Hardware: If you wish to develop for both Mac and PC, you should ideally have an Intel-based Mac (Mactel), an Intel machine running Windows (use Bootcamp if you have a Mac already), and a PowerPC mac. If you don't have a powerPC Mac, be sure to pay attention to endian issues if you need to write save files. There are also some bugs that will appear on powerPC but not Mactel. Ideally, you should acquire a used powerPC machine (e.g. Craigslist).
You can also make your Photoshop plugins compatible with other hosts (e.g. Irfanview, Paint Shop Pro). I've posted my experience over at the Photoshop SDK forum.
Try the aforementioned Photoshop SDK forum or the SDK mailing list.
This website is run by Glenn Chan.
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