Remmina is a remote desktop software client for POSIX-based computer operating systems. It supports the RDP, VNC, NX, XDMCP, SPICE and SSH protocols.

PAC is a SecureCRT/Putty/etc (connections manager with automations) equivalent for the Linux world.

KRDC is a client application that allows you to view or even control the desktop session on another machine that is running a compatible server.

NoMachine is a remote connectivity and network computing software.

Apache Guacamole is a clientless remote desktop gateway. It supports standard protocols like VNC, RDP, and SSH.