For the kind of thing that I've generally done, one of those black sanders has been adequate -- just a little sanding between coats of paint, sanding down spackel after patching a hole, that kind of thing.
But if you are someone who does a lot of little project, you might want to invest in a small electric hand sander. My mother has one of those Black and Decker Mouse sanders. Not only is it kind of adorable, it's faster and easier than using a block sander while still offering a lot of control for the user. It's relatively idiot-proof.
I don't even have to buy one -- I can just borrow hers for my next project!