Depending on where your work, the skills needed may be different.
A few come in mind:
instructional design basics (behaviorism vs constructivism approaches, cognitive science)
information/message design, interaction design, interface design
graphic design basics
media production- graphics, audio, video
programming- lectora, flash, javascript and other web technologies
If you work with multiple teams with multiple expertise, you probably don't need to know as much.
Also the books recommended by Ben http://www.eproficiency.com/page2419.html is pretty good. You should be able to find all the info you need from them.
HTH
