Adding about 1/2 cup of mashed potatoes for every 3 cups of flour in the recipe seems to be about as high as you'd want to go. Potatoes don't have gluten, so adding too much would result in a dense, heavy bread. I like to reduce the water in the recipe by about 1/4 cup for each 1/2 cup of mashed potatoes, since they have a lot of water in them. You can use the basic bread recipe for this, or any other one you like. If you don't have mashed potatoes, add potato flakes in place of some of the flour. Even using the water you cooked potatoes in, as the liquid in your bread, will help with moistness.
You can make a 'fully loaded' potato bread by adding some sour cream (reduce your water again!), bacon pieces, cheese, chives, or whatever else. See TheFreshLoaf for one version of this. Just remember- adding high-water ingredients- like the mashed potatoes, or sour cream- will require you to either reduce your water, OR add more flour. Adding mix-ins, like the bacon, cheese, chives, etc, doesn't affect the dough. However, it's easier to knead the bread without them, so add them last.
Now- for more options-
Use other purees! Use up to the same ratio as above, for the same reason. This will work best if the purees are warm, to help the dough's yeast grow. 110-120 degrees F is ideal.
-Mashed pumpkin- plain (which you won't taste), or with some cinnamon and pecans added!
-Mashed sweet potato
-Pureed fruit - good for using up overripe things, or old bottled fruit that's turning darker (as long as the jar's still sealed). You might want to reduce the sugar in the recipe; too much actually slows down (but doesn't kill) the yeast.
-Pureed vegetables- I've used green beans (up to 1-2 cups puree in the 6-loaf batch), pureed corn, chiles, olives, carrots, etc. Unless you want that particulat flavor, though, its best to avoid the cabbage/broccoli/cauliflower family. Just remember to reduce salt if you're using salted veggies, reduce by 1/4 tsp. for each cup of puree.
-Mashed cooked beans or other legumes.
-Cooked hot cereal- this includes leftover breakfast oatmeal, cooked 7-grain cereal, Cream of Wheat, cooked millet or amaranth, or whatever you happen to have. I've even added an abandoned, soggy bowl of cold cereal before.
This is a good way to use up little bits of (clean) leftovers- imagine the flavors of bread you can create! Just keep in mind that if you add something that has meat in it, you'll need to refrigerate the finished bread.
What things have you added to bread?