Hint: You Don't Need Much
The first three things on my list are only necessary if you will be working outside of the home or desire to have someone else feed your baby every now and then, and the last three are optional. You may find that breastfeeding is pretty darn cheap.
Moms who will be returning to work at any time during the first year will likely want to rent or purchase a breast pump. Moms who will be staying at home may want to have one on hand, as well. This will likely be your biggest expenditure as a breastfeeding mom. Do your research while you are still pregnant. Find out what machine your co-workers and friends used and peruse online reviews.
To go along with a breast pump, you'll need to get some sterile milk storage bags or bottles. If you want to build up a supply for your freezer, make sure you get plenty in advance.
To feed your expressed breast milk, pick up some bottles. Many state they are almost as good as mom, and a million different nipples, but you may have to experiment a little after your little one arrives to see which one she likes the best.
In my opinion, special nursing covers are not necessary and will often make you look like you are wearing a pretty tablecloth, as they can be pretty big. A light receiving blanket will do the job if you like to use a cover, but keep in mind that there are no laws stating you cannot nurse your baby in public -- even if you don't have a cover. Children who see women nursing in public will grow up thinking breastfeeding is a totally normal part of life, which is a beautiful thing.
An actual nursing bra is nice to have for easy access, but some moms (myself included) opt to use nursing tank tops, which have the added benefit of keeping your baby belly covered and out of sight. Special nursing tops are not really necessary, either -- you can lift your shirt or pull it down to nurse your little one. Dresses are more complicated so you may have to go the expensive nursing dress route, but if you get one low-cut enough, it will work regardless.
You might want to pick up a nursing pillow, like a Boppy, but they aren't totally necessary. They can make your nursing experience more comfortable, especially since even the smallest babies can get heavy after holding them for a half-hour feeding, but you can get by with your own pillows if you want. A Boppy is pretty cute, however, and make a good registry item.