Asterisk is an open source system for creating your own PBX. That's "private branch exchange" in telco talk, and it means that you could set up your home and home office with multiple extensions, conferencing, music-on-hold, voice mail, and all the features that you'd normally associate with an expensive telecomm system.
But as you might expect, it's a bit complex. So far, the best article I've found for getting an idea of what's involved, and what you might want to do is Kerry Garrison - Building your own PBX
I picked this tip up from the ever-helpful XTension discussion list, by the way.