Alfred is a free Android app (a revamped and relaunched version of "Web of Cam Internet") which turns unused smartphones into capable surveillance cameras.
It's all very easy to get running. Run the app on the phone you'll be using as a camera, and log in with your Google account. Run the app on the viewing phone, log in with the same account, and you're done.
At a minimum you can watch the feed, switch between the front and rear cameras, maybe take snapshots. There's support for multiple cameras and viewers, and video quality is adjusted automatically to try and maintain performance on slower connections (although this doesn't always work well).
Alfred provides two-way audio, too. You can hear what's going on, and join in, anything from singing to your baby to shouting at intruders.
No time to monitor the feed? No problem, Alfred's simple motion detection system will send you a notification whenever something happens.
The ads can be annoying but Alfred is a fun app with a good feature set which mostly works very well.