1. The Kitchen
If you work more than you cook, dedicate some of your counter space to a workspace. If you can't give up any of that precious counter, invest in an inexpensive kitchen island or cart (IKEA has some great ones, like this $59.99 model that's perfect for a laptop).
BONUS IDEA: You'll get great use out of multi-tasking your kitchen and office storage.
2. The Dining Room
Again, for the folks who's idea of "home cooking" is eating Chinese takeout on the sofa, use any available eating space—whether it's a formal table or just a tiny nook—as an office. A table and chair is a table and chair, right?
BONUS IDEA: The fact that your dining area is probably lacking in both power outlets and connectivity means you have an excuse to get crafty with cord control and get started making your place completely wireless.
3. The Closet
Kill two birds with one stone: Donate a bunch of your lesser-worn clothes to charity and make room to transform your closet into a home office. With the retail market going modern, there are plenty of small desks to be bought. Or, you can just make use of custom closet shelving, like in image directly above.
BONUS IDEA: If your closet is less walk-in and more peek-and-feel-around, try a desk on casters, like in this rolling hideaway closet office.
4. The Porch
If you've got an outdoor space, chances are it's the least-used room in your home. What's that? That's because it's below freezing outside right now? Well close it in with walls or super-thick curtains and wire up a few space heaters to get a cozy (if not comfortable) home office.
BONUS IDEA: Since you might already have motion-sensor lights installed out there, you'll be saving money with a super-efficient lighting system.