There are several good ones out there, Rudi. Sort of the standard is Kelty's FC 3.0 Child Carrier ($230). It has a lot of nice features, such as a kickstand that deploys automatically when you set the pack down. Comfortable for the child, with a shaded hood and adjustable seat, and it has a decent suspension for parental comfort as well. It also includes little storage nooks for the things a little kid needs.
Another good choice is the Deuter Kid Comfort III backpack ($280). Not dissimilar to the Kelty, but a little nicer suspension with a stronger frame, a bit better belt/shoulder strap arrangement, and the usual things such as accessory pockets and a weather hood for the occupant. Parents who have tried it say its easier to load and unload, too.
For a little more casual usehiking plus trips to the local farmers markettake a look at the Sherpani Rumba Superlight ($160). Its a slimmed-down carrier, with good features for youngster and parent alike, but in a lighter frame than some of the other packs. That means it wouldnt be quite as comfortable for a long day, but for quick jaunts or around the town, it might be what youre searching for.
Be sure to spend some time with these packs, trying each one on, loading and unloading your youngster, getting a feel for what works best. And then go from there.