When Less Is Less

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The TwitterPeek, a little blue device that looks like a pocket calculator with too many buttons, does nothing but manage your twitter streams. It belongs to an electronic breed that rarely succeeds but never quite disappears: single-purpose devices. From the days of the Apple (AAPL) II on, general-purpose computers have mostly sent more specialized products like these packing. Versatile smartphones, likewise, have demolished the market for Palm's (PALM) and other nonphone PDAs.

Yet some single-purpose devices survive and—in the case of Amazon.com's (AMZN) Kindle and other electronic book readers—even thrive in a multipurpose world.