I've actually been looking into these XLR camcorder adapters. Here's a summary of what I've found:
Beachtek: Welcome to the new BeachTek website! - Mounts to camera. Various passive adapters. Some with phantom and audio meter. New unit with preamp (DXA6HD).
Camcorder XLR Audio Adapter/Preamp: Buy Direct and Save from juicedLink - Mounts to camera. All include preamplifier. Options include phantom, 2-4 XLR inputs, audio meter.
Video Backgrounds, Studio 1 Productions Video Backgrounds, Royalty Free Music, XLR Adapters, Zoom Controllers, Stock Footage, Animations and More - Doesn't mount to camera, and no phantom or preamp option. Option for 3 XLR input. Plastic case.
Sign Video online catalog of video production, editing and duplication equipment, factory direct. - Mounts to camera. No phantom or preamp option. 1 and 2 XLR input options.
The juicedLink website has an interesting demo video, showing how a preamp can reduce camcorder hiss while recording fine audio detail:
juicedLink / Beachtek Demonstration Comparing Performance (watch video)