Official Intellivision Classic Videogame Website
Freeware emulator for MacOS and Win32.

Bliss
Features Bliss, an open source emulator for BeOS, Linux and Windows, and BlissJ, an open source emulator for the Java Runtime Environment which is also available as Java Applet.

jzIntv
Open Source emulator for Linux/SDL, MacOS, Unix/SDL, Win32.

Nostalgia
A freeware emulator for Windows.