Zacuto EVF Electronic Viewfinder
I have used the Zacuto Z-Finder Pro optical viewfinder for several years, and I consider it an essential accessory (about $375).
The Zacuto EVF electronic viewfinder (about $641) is an electronic viewfinder targeted at video users, but could prove very useful to still photography as well. Read me review of Zacuto EVF electronic viewfinder. In short, its focus peaking feature is of considerable aid to accurate manual focus in Live View mode.