MiSTer FPGA Project - This is an open source project, that hosts a multitude of console and computer cores. This is as close to the original hardware as you can get and since the project is open source it will get better and better as the years go by. There is a Colecovsion core along with Super Game Module support also. Check it out.
There are new and old Colecovision emulators. Quite frankly it been a crap shoot to track down really good ones. i'll be updating this page as things shift around with compatibility and such.
CoolCV by nanochess - This is by far the most compatible of all the software emulators. It supports large ROM files as well as Super Game Module titles. Try it out! You can take PNG screenshots, and record gameplay video. Excellent overall.
BlueMSX by Daniel Vik - This emulator also emulates the MSX computer from Microsoft as well as the Colecovision.
There are many other Colecovision emulators out there, but quite frank the support on them has stopped. The two above are your best bets.