When a CEO leaves a company, usually the first thing that happens is that the sitting Chairman is appointed as interim CEO. Their job is to steer the company until a full-time CEO is hired.
What do companies do when their CTO leaves? Usually get by without one while they begin the search for a new one. Something that I've experienced recently at both my old and new companies is an Interim (or Transitional) CTO someone to take care of technology until a full-time CTO is found.
An interim CTO doesn't start coding software nor configuring servers. Their job is to provide technology representation for the existing team. Technology teams need an advocate, and can feel completely unrepresented without one. Interim CTOs don't need to come up with a technology strategy, but they do need to mind the store, including shepherding in-play projects, running the budget, and executive communication.
iVillage had an interim CTO when I arrived, and his help has been invaluable to me as I got my head around the extensive technology footprint here. Its made me more effective. I could choose what to focus on first, fully knowing that anything I wasn't focused on was being taken care of by someone else. I recommend a 3 month overlap as ideal.
So, for any company losing their CTO, get an Interim CTO. There are plenty of talented consultants/ex-CTOs out there to help you, use them.