As I answer this question, I'm assuming that you're looking for a nursing cover-up, not necessarily a wrap-style baby carrier (like the Moby).
I found out about this product from a friend who uses it, and I've grown to love it too. Look up Hooter Hider or Bebe Au Lait. (They're the same product, manufacturer, and price -- just different names/ labels. The original name is Hooter Hider but I guess some people didn't want to wear something with a label that said Hooter on it.) It's kind of pricey for something so simple, but worthwhile for me since I don't sew. It's definitely easy to use and big enough to provide coverage. It has a little terry cloth pocket in one corner that I use to stash my breast pad while I nurse, and the pocket is also good for mopping up little messes. Best of all, the neckline has a wire in it that holds it away from your collar so that you can look down and see your baby.
It's saved me a lot of money because I haven't had to buy any nursing clothes other than bras. And, it's saved me a lot of trouble for times when I fumble with switching sides.
The only downside is that it can get a little too warm sometimes, but I think that just would come with anything that's thick enough to be opaque.