Android
Android devices can be used as network cameras themselves. There are various apps for Android that can provide a video stream, and some that simply record videos which can be imported into a surveillance system.
Apps
It is recommended to use apps from a trusted source. Some apps will serve ads, others will pull your personal information. Choose wisely.
F-Droid
Provides a repository of free and open source software for android.
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Wifi Camera Supports Mjpeg and audio streaming. Built-in web service to see the video via browser on a PC.
Spydroid
Mobile WebCam This application provides an MJPEG stream, from its web server. Does not work on all devices.
CosyDVR This application records Video to a SD card.
Rooting
By rooting your android device, you can install another operating system such as Debian, which will enable you to use that operating systems full repos.