Having been through it once without one, I would LOVE to have a doula at my next childbirth if I can afford it. Basically a birth doula is part advocate, part coach. The doula's responsibility is to make sure you have a good birth experience (as opposed to the doctor, whose responsibility is to deliver a healthy baby). She's* the person who knows your birth plan and isn't all hopped up on painkillers and hormones, and has experience communicating with doctors and nurses under stressful circumstances. She's also the person responsible for making sure your family either stays close or stays out of the way depending on your preference, and also will coach you through labor and transition with breathing/stretching/whatever you've discussed ahead of time.
*I suppose technically there could be male doulas, but I've never met or heard of one. Would he be a doulo?
My neighbor is a postpartum doula - she'll come to your house and help you learn how to breastfeed and make sure you're taking care of yourself, etc.
TMBG are (or is?) doing a Repeal Day concert.
Aw, man! Not in Brooklyn!