We spotted this armoire (far above) on This Old House. While we were initially drawn in by it's feminine style and stellar organization, we came to appreciate its other creative feature: a fold-down desk built in to the door.
You can get the same look from any old bookshelf, including that one you've had in storage forever. Or, if you want to buy new, we think this cheap IKEA BILLY bookcase ($49.99) would be a perfect start. Whatever you use, just make sure that it's anchored into the wall once it's placed in the room. You won't want this tipping over with the extra weight at the front.
From there, all you'd need to do is add a set of doors. Grab the ones that come with the BILLY line ($30.00) and follow the included instructions for attaching them. Or, if you're using an old bookshelf unit or something you picked up, follow these video tutorials on eHow for making bookcase doors and installing hinges.
Once the door is on, you'd need to custom-fit a drop-down desk. It sounds tough, but with some hardware store hinges, lightweight wood and this tutorial from HGTV, you'll be on your way to the perfect small-space workspace solution.
After the desk is attached to the door, you might find that you need to trim down the bookshelves to make space for the folded-up desk inside your bookshelf unit. That part should be easy for a DIY pro like you. We're going to assume you don't have room for a saw if you don't have room for a desk, so just make a trip to a nearby home store to see if they'll trim the shelves for you. Just remember: Measure twice, cut once.