On a vintage mac I never want to miss this:

PopChar, great enhancement to the use of any keyboard (still available for OS X)

Copy-Paste, the way a clipboard should be organised (also still available, but the current approach is not as convincing as the original was - CopyPaste X is overkill, in my opinion)

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Something pretty funny was the extension PowerOrgasm (sound stolen from "Harry and Sally"). On a running PowerBook, just plug in the power connector …