This is going to come off nauseatingly optimistic, but here's my take:
Made in Space is working on producing ZBLAN optical fiber in orbit, for sale back on Earth. [1] ZBLAN is an alternative to normal silicate fiber which has much higher theoretical performance limits, but worse performance in practice due to gravity induced crystalline impurities. Producing it in microgravity should allow it to reach its full potential.
They haven't launched the first batch yet, but their claim is that because optical fiber doesn't weigh much, and this higher quality fiber is so valuable, it's actually profitable at current launch costs to do this manufacturing in orbit.
The linked article ends with a quote from the CEO: "We believe in-space manufacturing of goods valuable to people on Earth will soon drive significant commercial activity in space, perhaps one day creating a space-based economic boom." Consider the lithography processes our chips are made with, or the single-crystal turbine blades that help modern jet engines and rocket turbopumps work as well as they do. There's an enormous variety of precision processes that could benefit from operating under an ambient acceleration between 0 and 1g.
Orbital manufacturing is probably the most believable short term goal for commercial space, but there are a plenty of extremely exciting opportunities out there:
- The Mars colony will be our stepping stone out into the asteroids and the rest of the solar system. The thin atmosphere and lower gravity makes lifting mass from the Martian surface about an order of magnitude less difficult than lifting from Earth's surface. The difference is so extreme, that if you assume commodities will someday cost about the same on both planets, structures in Earth orbit would actually be cheaper to build and launch from Mars and then tug over to Earth.
- The potential wealth from mining other bodies is unfathomably huge. A small minority of asteroids out there are actually chunks of proto-planet cores unleashed during the violence of the early solar system. A single rock like that has more accessible platinum, rhodium, and other heavy metals than all known accessible deposits on Earth's surface. This fact becomes a little less surprising when you learn that all of Earth's accessible deposits are nothing more than impact sites from rocks like this. The "original" heavy metals of Earth all settled deep inside the core while the planet was still molten, far too deep for us to ever reach. These heavy elements are unbelievably useful. The state of the art in a huge fraction of the "hard" problems in chemistry boils down to "We can do it with platinum group catalysts, but those cost too much for industrial use."
- Orbital superstructures. Made in Space's Archinaut project [2] is an early effort to free satellite designers from the size and weight constraints of delicate folding structures riding inside a single rocket fairing. It's still early, but once launch costs drop and/or we start doing heavy in-situ mining, we can reasonably start building kilometer scale stations. Enormous solar arrays to power all our new orbital industry, or 5,000 person orbital ring cities with a relaxing level of partial g and perfect climate control. I'd love to live in a place like that.
My favorite analogy for this is the settling of America. Settling the new world in sailing ships that took a month to cross the Atlantic was a terrible investment with hardly any foreseeable return. But then we made better boats, and new communication systems, and all sorts of other new technologies which created exponential returns from the colonies. The west still leads the world because of the unimaginable profits it made off the Americas. Settling space, and particularly Mars, is the biggest opportunity we've had since the 1500s.
[1] http://www.madeinspace.us/projects/fiber/
[2] http://www.madeinspace.us/projects/archinaut/