A rug. All I can say is, why didn't we get a rug sooner?! It's the best. I've always hated our carpet, but the rug brightens things up and makes the room look bigger. It just makes the space look like a space. If you're on the fence about getting one, get one! Please. You won't regret it. Ours is this Moroccan rug from Overstock (similar ones here and here).
A house plant. It's nice to have something alive and green indoors, even if it's small and as low maintenance as possible. I couldn't resist this Meg A. Myers terrarium (especially since I got a deal on it at the Renegade Craft Fair -- it has a tiny crack in one of the panes) and added a succulent and cactus. My goal is to keep them alive for at least three months, ha!
More floor space. We got rid of our coffee table, and I love the extra room we have now. After sitting at a desk all day, it feels so good to come home and stretch out at night.
Next up... throw pillows!