TightVNC

TightVNC is an incredibly powerful, deeply entrenched open-source veteran that mathematically pioneered the "Low-Bandwidth VNC" market. It doesn't rely on massive cloud brokers; it is a pure, decentralized mathematical implementation of the RFB (Remote Frame Buffer) protocol. It is the absolute weapon of choice for hardcore system administrators and embedded systems engineers who demand a terrifyingly lightweight, 100% free tool that works flawlessly on slow, archaic network links.