My wife and I make dinner in our slow cooker a couple days a week.
You can do a lot of stuff in the slow cooker, but most of what we do is fairly similar to what my pal Punisher described above: get a good hearty protein (pork shoulder, lamb shoulder, short ribs, or a whole chicken), add a chopped onion or two, maybe some garlic cloves, carrots and/or potatoes. Most cuts of meat can be cooked "dry" but you can also add some flavor with a little cooking liquid: broth, wine, beer, hard cider. A personal favorite of mine:
4-5 pound lamb shoulder
2 large apples
2 large onions
1 cup raisins
1 can/bottle hard cider
An optional step, but one that I think really adds something, is searing the meat in a skillet before you put it in the slow cooker. Just 3-4 minutes on all sides to brown it and get a little crust. Then drop it into the slow cooker, cover with chopped onions, apples, raisins, and the cider. Cook on low for at least 5 hours (although more really won't hurt; you can leave it in as long as 10 hours, really).