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I've seen some forum threads where a user has removed a widget without knowing how to get it back, because he/she doesn't know that "the thing you use to switch desktop" is called 'Pager' for example.

My idea is to have a "Recently removed widgets" category in the Add Widget dialog. It will list x of your last removed widgets (I don't know how many would be optimal), so you don't need to browse though the list to add them again if you change your mind. As a bonus, it'll also restore the widget's configuration, which means that you'll basically get the same widget as the one you removed.