I've used a Kelty backpack, a BabyTrekker, an Ergo, and a Patapum, and like the Patapum best (followed by the BabyTrekker).
These back carriers were also recently in these threads:
http://www.mothering.com/discussions...d.php?t=220904
http://www.mothering.com/discussions...d.php?t=222995
http://www.mothering.com/discussions...d.php?t=217498
These back carriers were also recently in these threads:
http://www.mothering.com/discussions...d.php?t=220904
http://www.mothering.com/discussions...d.php?t=222995
http://www.mothering.com/discussions...d.php?t=217498