There are numerous others in this category but TeamViewer wins outright, so I haven't listed any others.
Whether connecting to your aged relative or a customer in need, if you need to take control of their screen, TeamViewer is the right tool.
Meanwhile, an excellent free tool for small teams to perform teleconferences, video chat, with screen sharing has emerged:
My book "Choose Your First Product" is available now.
It gives you 4 easy steps to find and validate a humble product idea.