(on Wikipedia)
https://virtuawin.sourceforge.io/ (gone as of 2016-02-05)

A virtual desktop program.

Absolutely mandatory.

I used this on Windows XP and Windows 8.1. Windows 10 has reasonable virtual desktop functionality built-in. See using Windows 10 virtual desktops.

Alternatives ∞

• Windows 10 has its own feature. See using Windows 10 virtual desktops

• Sysinternals Suite has Desktops.exe