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By Douglas MacMillan
Twitter hasn't made a penny—but that's not stopping a whole class of entrepreneurs from trying to build their own businesses on the back of the microblogging pioneer. Twitter makes its code available to outside developers who in turn can create their own tools that sort, analyze, distribute, append ads to, or otherwise interact with the millions of 140-character messages posted to Twitter each day.
From Twitter browsers to ad networks and multimedia tools, third-party programmers have dreamed up dozens of original ways to experience Twitter—and to make money from it. This BusinessWeek slide show tunnels through the many layers of the new Twitter ecosystem.