Depending on where you want to adopt FROM, I can tell you that you can't go wrong with Wide Horizons For Children. We adopted our son from S. Korea thru them, and they are professional, ethical, and caring. They also work in quite a few countries other than S. Korea.
Another agency, Carolina Hope, who we used to adopt our daughter from Guatemala, is also pretty good, but that was so many years ago, and I really haven't keep up with them.
What country are you considering? I would love to adopt again. Our youngest is turning 2 on Halloween, and I am feeling a strong urge for another baby! We adopted her domestically. We might try that again, but Haiti, Ethiopia, and Taiwan are all calling to me!