Modified my camera so that it can take 90 degree photos. The porthole effect was an added bonus. I've had several occasions where I've wanted to take horizontal photos while looking down at the viewfinder (i.e. taking a photo of a crawl space while looking down a hole) and this suited that need nicely.

This was built from some simple household items... a plastic water bottle, a mirror, and some tape. The camera lens fits through a hole in the side of the water bottle. The mirror is inserted through a slit on the opposite side at a 45 degree angle. The far side of the water bottle is then chopped off. The longer the water bottle, the more the hole. I eventually chopped the bottle down nice and short to increase the field of view, but I snapped a few photos around the house while the bottle made a good porthole.

It's not easy taking a photo of the modified camera with the modified camera, but the shot came out pretty cool none-the-less.