A WebTop is a web browser based desktop environment, designed to provide the functionality and, to the extent possible, the usability of a traditional desktop environment. 
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See <a href=http://en.wikipedia.org/wiki/Web_desktop>Wikipedia</a>.
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Related items: <a href=http://en.wikipedia.org/wiki/SaaS>SaaS</a>, <a href=http://en.wikipedia.org/wiki/Web_2>Web 2.0</a>