My favorite gimmick band by a wide margin would have to be Optiganally Yours. Every song prominently features the Optigan, a cheap-ass keyboard from the early seventies that played pre-recorded optical discs. The instrument is very limited and generally sounds like hell, but somehow they managed to create two albums worth of surprisingly excellent songs with it.
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On a similar note, I get a kick out of The Moog Cookbook (in small doses).
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