Turn an old phone into a baby monitor
Baby monitors are one of those purchases that feel slightly absurd once you notice the old phone in your drawer has every part they do.
What it does well
Prop a spare phone so it sees the crib, and the browser can do the core job: show the live view, meter the microphone for a cry, watch the camera for movement, and alert you — a flash, a beep, and a timestamped line in an event log so you can tell whether the last hour was calm. Turn on night mode so the screen dims to a soft dark instead of glowing like a nightlight, and it holds a wake-lock so the phone keeps watching instead of sleeping. You can even have it auto-record a short clip when something happens. All of that runs in the tab, on the phone, for free.
And the second phone in the drawer
For a long time this note ended with an honest limit: no server, therefore no watching from a second phone. That limit turned out to be only half true, and we've since retired it. A server is needed to introduce two phones, not to carry their video — and the introduction fits in a QR code held between them. So the drawer's other occupant can now be the parent unit: it scans the nursery phone's code, shows a reply code back, and from then on the picture and sound travel phone-to-phone across your own wifi, never the internet. The full story of how that works is in two phones, one wifi, zero cloud; the short version is that solo mode remains the simplest setup, and the two-phone handshake is thirty extra seconds when you want the crib in your pocket. Either way it stays serverless, free, and offline-capable after the first load.