The Shadow hot shoe adapter for the NEX-5 and NEX-5n cameras allows you to add a wireless trigger system or PC Sync cord connection for your off-camera flashes. Plugs into the accessory slot and provides a basic "hot shoe" and PC Sync outlet. US$80 at NEX Proshop.
Want a wired remote release for your NEX-7? See these DIY instructions.
Ownuser has come up with a vertical grip for the NEX-7 camera. It also makes a grip for the NEX-5 models, as well.
Kipon (Shanghai-based maker of accessories) makes a Nikon lens mount adapter (Kipon Shift NIK-NEX) for the NEX E-mount that provides up to 12mm of shift. Shift allows you to correct for distortion due to perspective (typical of shooting upwards with a wide angle lens. Kipon also makes a tilt Nikon lens mount adapter (Kipon Tilt NIK-NEX). Tilt allows you to change the focus plane. It's how those "miniature effects" are done (by putting on a sliver of the sensor at the focus point) and it's how landscape photographers get large depth of field (by tilting the focus plane slightly ala what the big format cameras do. You're likely to have to perform an eBay search to find one, though.
PalmGrip is a US$99 combination grip and Arca-style plate for the NEX-7. At 80g, the black aluminum grip allows access to the battery chamber and card slot without removing the grip from the camera.