Two people connect via a 4-letter code. Each round (25 min), you press P to declare you're working. Round only counts if you last 20 minutes without giving up. See your rival's status in real time.
Solo mode also available.
No accounts, no apps. Single binary: curl -L https://github.com/aragosta-bot/pomodare/releases/latest/dow... -o pomodare && chmod +x pomodare && ./pomodare
Source + landing: https://aragosta-bot.github.io/pomodare/