I tried an Arduino based one and the Raspberry PiCam. PiCam was far better quality. The challenge is getting it to stream real time which typically requires a hardcoded IP address. Easy if you have a router, hard if you live in an apartment complex.

The s/w for both can be largely cut/paste from the templates.

Of course you could also just buy a dropcam or similar device, which streams video to a site and then the mobile app watchest that stream, with a ~3 second delay. But that's not really fun