Raspberry Pi camera modules
My main usage of the Raspberry Pi environment is its camera module. It started with the wish to document the construction of our new family home as a timelapse video. I tried other camera systems like the XiaoYi Actioncamera but the flexibility and image quality of the Raspberry Pi camera module was by far superior. If you're interested in the mentioned timelapse project, I will write it down and publish it in the near future hopefully.
There are multiple official and third party camera modules available that fit the CSI connector on your Raspberry Pi.
One sidenote: I'd like to add some photos of the actual camera modules, but first I have to prepare something that helps me with the lighting. So at the moment I added images of my simplified CAD objects that I use for desigining my other parts so you can get an idea of the actual camera module.
So far, there are two official camera modules on the market, the older v1 with a 5MP OmniVision OV5647 sensor, and the newer v2 with a 8MP Sony IMX219PQ sensor starting 2016. Both are available with and without an infrared cut filter (NoIR = no infrared cut filter). The PCB of the NoIR version is black, the other one green. The M7 lens mount is glued onto the sensor board and cannot be removed without risk. The sensor board itself is sticked onto the PCB with a glue pad on its backside. It could be removed if you carefully detach it from the little connector and the glue pad.
The lens seems to be plastic and not UV light resistant. After about a year outside the lens got slowly milky. You could place an UV light resistant optical glass in front of it or modify it to use an UV light resistant M12 lens. In order to test that, I ordered some optical glass discs that I will put in front of the original lens to solve this issue for projects where I don't want to use a M12 lens. They finally arrived but I haven't had the time to create a socket that will allow me to mount it in front of the lens. See UV light protection glass disc on Raspberry Pi camera module for more details.
Third party modules
There are some third party camera modules available that are far cheaper in price, but sadly in quality too. The sensor is the same as in the v1 (5MP OmniVision OV5647) but the lens seems to be of a much lower quality. The image is more grainy and a little blurred. You can still use it for surveillance cameras or other areas where it doesn't have to be crystal clear but you probably would not use them to capture holiday moments. The lens has the same UV light degrading problem as the original lens. There are no third party modules based on the newer 8MP Sony IMX219PQ sensor.
Beside the copies of the original with its small M7 lens, they offer versions with a M12 mount and lenses with different focal lengths. But the only third party version I find really interesting is the smaller mini version for the Pi Zero with its tiny overall dimensions. It is far less bulky in comparison to the PCB of the big versions.