My cheap and cheerful, but not very secure homebrew solution is a used smartphone, then load any of the motion-detection apps onto it, plus an FTP server app. Then place the phone anywhere within Wi-Fi reach. Run a script once a day on my home server that downloads and deletes the videos from the phone via FTP, and also deletes that footage after 30 days. So the "system" can run indefinitely without running out of memory. The old phones just need to be rebooted once in a while for some odd reason.