I just had an experience with this yesterday. I just put a little olive oil on it and toast it. First I had cut a slice of bread, then just spread some olive oil on the slice on both sides, and toss it toaster. Worked really nicely for me.

This is a little off topic, as it is not a method to soften hard bread. But the best thing to do with it is to make breadcrumbs! Just give it a few minutes in the food processor and store. They'll keep for a good long time.

A baker I worked with used to just flick cold water on the loaf, wrap it loosely in foil and bake it until it came back to life.

My thought is similar to @Ivysfeast. I always thought the method was to run it (quickly!) under the faucet and toss it in the oven til all is well again.

@LauraJane I do the same- a quick pass under a running faucet and then I toss it in the toaster oven until it's ready.

America's Test Kitchens just had a segment about this recently. There is a difference between bread that has been sitting around for a few days (it's hard and dry because of crystallization, which you can reverse by wrapping the bread in some foil and reheating it) and bread that has been dried out with heat - which needs to have moisture added to reconstitute it. The days-old bread will absorb some extra moisture, yes, but that's not the crux of its problem.

Slice it thin and heat it in a skillet with olive oil and garlic.

Since I get a lot of my bread/rolls in a paper bag, I usually just dampen the bag and throw it in the oven on a low temp for ten minutes or so.

Or just make it into bread pudding. :) How about squash and carrot bread pudding for dinner? The bread soaks in some milk before baking so hard or stale bread is actually the best.

Or go the Catalan route and make pa tomaquet. You just smear it with tomatoes, olive oil, and salt and then put a little cheese, ham, or whatever you have around on top. yum.

I don't like eating hard as rocks bread so this is a great tip. My tip to prevent your bread from getting hard and staying soft for a few more days, don't keep it in the paper bag. Take it out and wrap it in plastic.

I use the oven method, since I really don't like the microwave for much. I don't ever seem to hit that sweet spot of where the bread is perfectly warmed, I go from hard and dry to rubber with no stop in the middle. The towel, foil and paper bag all work quite well in the oven.

My grandmother always used to just run it quickly under water, then place it on a towel on her heater/radiator- in the winter time, that is. We'd still be warm, not waste unnecessary energy, and the rolls/bread would be soft and ready for dinner.

The problem with wrapping the loaf in plastic is the entire loaf becomes soft, sacrificing the crust. Some people store the bread on the cutting board, cut-side down for larger loaves.

my dad has developed the following method for day-old baguette:
make the crust surface wet by stroking it with wet hands, avoid the parts where the crust is broken. bake for 10 minutes in a 110°C/230°F hot oven

I use the "it must be time for garlic bread" method. So, I guess that's the same as adding moisture, wrapping it in foil, and baking it.