Remmina - Remmina is a remote desktop client written in GTK+, aiming to be useful for system administrators and travellers, who need to work with lots of remote computers in front of either large monitors or tiny netbooks.
Wine - Open Source Software for running Windows applications on other operating systems.
Xpra - multi-platform screen and application forwarding system
CodeWeavers CrossOver - Don't buy a Windows license, don't reboot and don't use a Virtual Machine. Try a free trial of CrossOver to run your Windows software on Mac, Linux and Chrome OS.
X2Go - Get X2Go. Installing X2Go (client/server). Download X2Go Client (Windows installer (XP and Later), OS X 10. 9 and higher DMG, OS X 10. 10 and higher DMG or macOS 10.
WineBottler - WineBottler is an App to manage and wrap your Windows apps into convenient OS X apps.