FreeRDP is a free implementation of the Remote Desktop Protocol (RDP), released under the Apache license.
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. VNC and RDP is supported.
An open source UNIX client for connecting to Windows Remote Desktop Services, capable of natively speaking Remote Desktop Protocol (RDP) in order to present the user's Windows desktop. rdesktop is known to work with Windows server versions ranging from NT 4 terminal server to Windows Server 2012 R2.
PuTTY is a free, open source implementation of SSH and Telnet for Windows and Unix platforms with an xterm terminal emulator.