What is the ideal hydration for bread dough?
Bread hydration varies widely.  The "standard" bread using all-purpose (plain) flour has a ratio of water to flour weight (hydration) 60-65%.  Flour with a higher protein level, labelled as bread, strong, or high-gluten, tend to use 65% hydration.  Ciabatta and rustic breads generally use more water than normal.  The extra water gives them more large, uneven holes in the interior of the bread (called the crumb), and generally leads to a higher-rising bread. These wetter doughs are often referred to as "slack" doughs in baker's parlance.

Here are a couple sample hydrations from Hammelman's bread baking:


Baguettes with poolish, 66% hydration, all bread flour
Ciabatta, 73% hydration, all bread flour
Pain Rustique (rustic bread), 69% hydration, all bread flour
Country Bread, 68% hydration, all bread flour
Roasted Potato bread, 61% hydration, 85% bread flour / 15% whole wheat flour / 25% roasted potatoes
Whole wheat bread, 68% hydration, 50/50 whole wheat and bread flour
Semolina (Durum) bread, 62% hydration, 50/50 durum and bread flours


Mind you, it is quite possible to make a bread with even higher water content, if one is a skilled baker.  Wetter breads (70% hydration and up) generally cannot be hand-kneaded normally, and require a mechanical mixer, stretch-and-fold kneading with a spatula, or autolysis. Autolysis is when you mix water and flour before adding yeast, and then allow it to sit.  This allows enzymes in the flour to develop gluten before the rising begins, and can supplement or replace normal kneading.  

Another approach, called double-hydration, is to add only part of the water before kneading.  This allows you to knead the bread to develop gluten structure before it becomes too wet to knead.

For extremely wet breads, these methods may all be combined.  I'm looking right now at a double-hydration, mechanically mixed, autolyzed, poolish-using ciabatta recipe that sits at 76% hydration.