Apple's Widget Gallery has the largest widget selection I've seen. Of course, it's just the gallery and technical documents.
Dashboard Widgets provides the strongest developer community and boasts the second largest selection. The admins are quite responsive, putting up new submissions within hours.
Widget Developer can offer great tehnical assistance for developers, but the gallery is a little small.
MacUpdate will advertise widgets and has a decent gallery, but there is no dashboard developer community to speak of.
MacPrime looks pretty solid, but I have to run everything through translation pages because I don't speak German.
Dashboard Exposed and DashboardShare are both quite lean in both the gallery and the community, but people do download from the sites.