In addition to what the others mentioned, there's also paperwork (such as my nemesis, filing); the time you spend running to OfficeMax to buy a new chair or a box of paper; the time spent looking for medical insurance (and being aghast at what it costs for an individual); "casual" contacts that sometimes can be transformed into business networking but don't, always; doing your invoicing and bookkeeping (most freelancers I know are terrible at this). And that's just off the top of my head.

I spent years as a sysop of the CompuServe Computer Consultant's forum. I'm kind of familiar with this stuff. <g>