Here are a couple examples of opening a URL in a window when clicking on the zimlet from the left Panel.
This example shows how to handle a single and double-click and it is all done in the Zimlet Definition File XML (i.e. no JavaScript required):
ZCS 6.0:Zimlet Developers Guide:Examples:Panel Item Clicked XML - Zimbra :: Wiki
This example shows handling single and double-click events in the Zimlet JavaScript Handler Object:
ZCS 6.0:Zimlet Developers Guide:Examples:Panel Item Clicked - Zimbra :: Wiki
And as an FYI, here is a link to the Zimlet Definition File Reference:
ZCS 6.0:Zimlet Developers Guide:Zimlet Definition File Reference - Zimbra :: Wiki