The Mountain Dulcimer dates back to about 1780 or so, and the history of the instrument is extremely vague and open to much speculation. It seems to have developed almost simultaneously in widely separated geographic regions of the United States, although most of the older instuments were found in the Central and Southern Appalachians.
Mountain Dulcimers are found in a breathtaking variety of different shapes and sizes, and they are made out of almost any kind of wood imaginable(and some that are pretty hard to imagine!). These instruments are a testament to American ingenuity, in that the early settlers of the Appalachians fashioned their dulcimers out of whatever local lumber they could find and with whatever crude tools they had on hand. So Mountain Dulcimer construction is essentially a folk art: it does not have its roots in a long European tradition, like Violin-making or Lute-making(lutherie).
Today, this folk art is incredibly healthy, although it has certainly been influenced by our modern technological age in countless ways: from increased access to tools, to greatly increased information-flow, to some interesting and thought-provoking cross-pollination with modern Guitar building techniques.
So many of these websites will be places where you can see catalogs of mountain dulcimers, hear some sound samples, or read about a particular craftsmen's insights on what makes a great dulcimer.
However, there is lots of MUSIC being played on the mountain dulcimer these days. In fact, I would say that the dulcimer has undergone a magnificent "Renaissance" since about 1970 or so.(this is a VERY rough date!). So, there are some very talented instructors and recording artists out there touring right now, and some of these folks have websites where you can take lessons, purchase CDs and tunebooks, and get an updated schedule of their workshops and concerts.
To start with, there will probably not be any major category breakdowns, or "sub-categories". I'm sure the number of sites will grow quite rapidly, though, and eventually there will have to be sub-categories.