Zopim Chat offers a plugin for Zen Cart making it easy to offer online chat services to your website visitors.
You can see it in the lower right corner on our site. The control panel, what you use as the operator, is browser based so can be easily accessed anywhere you have a browser and internet access. They recently released a fee iPhone app as well.
For more details about this plugin, please see our blog post How to Use Zopim Chat for Zen Cart