Zimlet is a neat architecture concept. But presently it's available only along with ZCS. Since per the guide -

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While the Zimlet Specification was designed for the Zimbra
Collaboration Suite, the model could be easily be generalized for other Web 2.0/AJAX user interfaces.
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I think it would be useful to make it available as a separately downladable component under a flexible license such as done for ZTK. Are there any plans for doing so? If yes, any timeline thought of so far?