If you have several different cameras, these plates can be frustrating to fit. This 3D printed design on Thingiverse is compatible with most tripods. The design is free and you can print it out on your own 3D printer.
A focus ring for the Canon Rebel that comes with the kit is expensive or unavailable, so this user made their own version of the focus ring for an 18-55 mm lens for $17.
GoPro bike mount
To get your camera a point of view from your bike, this mount replaces the stem cap to install any Go Pro camera. It runs about $10 on Shapeways.
Ski goggles mount
This 3D printed mount, available for about $13 on Shapeways, attaches to your ski goggles to mount the Camports Evo camera. It's unobtrusive to your vision and securely mounts on the side of the goggles.
This $13 frame for GoPro can also hang from things (like a toy helicopter or drone). The controls can be accessed by pressing the frame (through the case). The shop has other GoPro frames available as well.
3D printed digital camera
Forget all those accessories, you can just 3D print your own digital camera with instructions and materials listed on Instructables, an open source platform for 3D printed items. The maker said this isn't great quality, but rewarding to print nonetheless.
3D printed pinhole camera
This pinhole camera was completely 3D printed and shoots medium-format 120 film in a square crop. The design was featured on Thingiverse, but it's now for sale for $50 on Tindie.
This photo was taken with the acrylic lens above, made with a 3D printer. The design is available on Thingiverse for free, and the actual lens can mount to many different cameras.
Tilt-shift lens adapter
These lenses are used to create a shallow depth of field or miniature effect for photographers. However, they're very expensive, even though they're quite simple. This page on Instructables has several versions of this lens adapter, and there's even a video tutorial to show you how to print it at home.