Check the Hawaii Ohana board (below) for LOTS of information.
But in a nutshell, Molokini crater (inner reef and backwall) and Lanai (Cathedrals I & II) are definitely places to dive. Charter ops will depend somewhat on where you stay, but basically in Kihei the most often recommended shops are B&B, Ed Robinsons, and Mike Severns. In Lahaina it's Lahaina Divers and Extended Horizons. If you want guided shore dives, definitely Shaka Doug in the Kihei area.
There are a couple of locals who frequent the board (Kidspot being one of them) who will shore dive with you as well.