I have no recommendations that are free-as-in-speech.
NoMachine[1] is free-as-in-beer[2] and it works nearly flawlessly across linux/windows/mac.
2: It's freemium with the following note: "If you need to create virtual Linux desktops on demand, want to limit the logins to SSH, or are using NIS, Kerberos or other enterprise functionalities, you need one of the NoMachine Enterprise products you can download"