May have found a possible bug. Let me know if you can confirm and I'll submit a bug report.
I added a module - Aggregated Twitter Feed (http://gallery.orchardproject.net/List/Modules/Orchard.Module.AggregatedTwitterFeed), enabled it and added a widget
to one of my zones. I then realized that this module wasn't what I thought it was...so I removed the widget and disabled the module.
When going to add another widget to a zone, the Aggregated Twitter Feed widget was still in the list. Since the module was disabled, I deleted it from my server. When I went to add another widget to a zone, the Aggregated Twitter Feed widget was again still
there. I looked in the database and found it referenced several times.
I'm thinking that if I disable a module that is a widget, this should prevent it from showing up in the available widgets list...correct? I'm not sure if that's module specific or if Orchard itself takes care of that action. If it's module specific, then
I'll contact the owner of the module...if not and I'm not mistaken about how that should work, let me know and I'll submit a bug.