PlainApp is an open-source Android app. Install it, start the server, open the matching web page in your computer's browser, and you can view and manage phone files and transfer them — no desktop client needed.
On the same Wi-Fi you can browse, upload, download and rename phone files, or view and play photos, videos and audio right in the browser. You can also view contacts, SMS and call logs, send texts, and receive phone notifications.
It supports peer-to-peer file and chat transfer between devices, with data moving mainly over the local network and no cloud account required.
PlainApp also works as a phone app itself: Markdown notes, an RSS reader, TV casting, a pomodoro timer and a sound-level meter.