ErgoAssessment is a free posture evaluation that runs in your browser. No account and nothing to install, and the analysis happens on your device.
Set your laptop or camera to your side, sit the way you normally work, and after a short video capture of your posture get an analysis of what's wrong and suggested improvements.
You get five checks: how far your head sits ahead of your shoulders, head tilt, back, arms (elbow angle and forearm slope), and legs when they're in frame. Each one shows a gauge with the comfortable zone marked and a short, plain piece of advice on what to change. Run it again later and it shows where you were last time, so you can tell whether the changes stuck.
Rather not use the camera? Upload a side photo instead. Made for anyone who spends long hours at a desk, from programmers to gamers.
I got into ergonomics through my home office brand, CreateComfy, but I'd suspected my posture was off long before that. Checking the way you sit yourself is very hard, especially when you're not an ergonomist. Even after I learned to sit better, the only way to check myself was a photo from my phone and a lot of squinting. Then, while writing about desk setups, I wanted to point people to a quick posture assessment and couldn't find a good one. So I built it. How you sit is the baseline everything else depends on, from the chair to the monitor height, so it's very useful people have access to such tools.