Rdesktop

rdesktop is an implementation of a client software for Microsoft's proprietary Remote Desktop Protocol. Rdesktop is free and open-source software, subject to the requirements of the GNU General Public License, and is available for Linux and BSD as well as for Microsoft Windows.
As of August 2013, rdesktop implements a large subset of the RDP 5 protocol. Unlike Remote Desktop Connection in modern versions of Windows, rdesktop still supports the older RDP 4.0 protocol used by Windows NT 4.0 Terminal Server Edition and Windows 2000 Server.

WWW Official Site

Feature set

RDP5 features supported:

Bitmap caching
File system, audio, serial port and printer port redirection
Mappings for most international keyboards
Stream compression and encryption
Automatic authentication
Smartcard support
RemoteApp like support called "seamless" mode via Seamless RDP
Network Level Authentication

Still unimplemented are:

Client redirection upon reconnect
Remote Assistance requests
USB device redirection

Support for the additional features available in RDP 5.1 and RDP 6 have not yet been implemented, although some have been officially documented on MSDN.

Use

Rdesktop is commonly used on desktop ReactOS and Linux installations to connect to Microsoft Windows running Remote Desktop Services. There are many GUI clients, like tsclient, Gnome-RDP and KDE Remote Desktop Connection (KRDC), which are graphical front-ends to rdesktop. The program has also been integrated into several thin client Linux distributions like Thinkstation and the PC TSC project, as well as some thin client appliances.