An exocortex is the part of your mind that exists outside of your brain. May also refer to the hardware and software systems that host that part of your mind.
At our current level of technology, a viable exocortex will most likely be hosted on some form of wearable computer. Such computers typically aren't too difficult or expensive to build. Steve Mann has instructions for building a "WearComp6". Complete models can also be bought from manufacturers like Xybernaut.
The most expensive and difficult to construct part of a wearable computer is the display. One need only browse Tek Gear to get confirmation of that. Wearable technology is just waiting for the invention of a cheap and rugged HMD. The Teleglass T3-F looks like an interesting option.
The development of the software component of a wearable exocortex need not wait for the invention of a cheap HMD.
Applications like the Remembrance Agent may already give us a taste of what services such software could provide.
Augmenting Human Intellect: A Conceptual Framework - Douglas C. Engelbart
Humanistic computing: 'WearComp' as a new framework and application for intelligent signal processing
Exploring the Exocortex: An Approach to Optimizing Human Productivity