ZenPayroll, whose service helps small business run their payrolls over the web, is a tiny startup in a field dominated by giants like ADP (ADP), Paychex (PAYX), and Intuit (INTU).

Yet in the past five months alone, ZenPayroll has gone from processing paychecks at a rate of $100 million to $400 million annually. The staggering growth suggests there is plenty of demand among small businesses for its easy-to-use payroll services.

The growth is also a reason why investors like Kleiner Perkins Caufield & Byers and General Catalyst Partners are sinking $20 million into ZenPayroll.”

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