TeamViewer [FREE PERSONAL] (win, mac. linux, android, iphone)
It is certainly the best known and the most used software.
TeamViewer lets you establish a connection to any PC or server within just a few seconds. You can remote control the PC of your partner as if you were sitting right in front of it.
New features such as VoIP, webcam and application selection will give you an even better, easier and faster desktop sharing experience.
Chrome Remote Desktop [FREE] (Win, Mac. Linux, Android, iphone)
Chrome Remote Desktop allows users to remotely access another computer through Chrome browser or a Chromebook. Computers can be made available on an short-term basis for scenarios such as ad hoc remote support, or on a more long-term basis for remote access to your applications and files. All connections are fully secured.
X2Go (Win, Mac, Linux)
X2Go is an open source remote desktop software for Linux that uses the NX technology protocol.
X2Go can be used to access PC desktops, but its main use case is providing secure standalone remote sessions via ssh.
The server package must be installed on a Linux host; client packages can be run on Linux, OS X, or Windows.
DWService FREE-OPENSOURCE (Win, Mac, Linux, BSD)
DWService – Remote Access, Remote Support, Service Desk, offers a service that allows access to remote systems using a standard web browser.