I've been testing the waters over at smashwords.com.
The general consensus online is that formatting books to Smashwords's standards is a nightmare, so I started out with my two free short stories in the Posthuman Cycle universe. Conclusion? It's much easier than advertised and far easier than formatting an ePub by hand.
You can download them for free here:
Works by Micah R. Sisk on Smashwords. Why Smashwords? When you publish there they will distribute your works (even free works) to other eBook retailers like Barnes & Noble, Kobo, Sony and Apple's iBooks. Assuming, of course, that your ebook formatting passes SM's premium review process, which both of my short stories did. So in the next week or so these short stories will become available on said retailers' web sites.
And once that's happened, I can publish them on amazon.com and (hopefully) coax them into matching the price (FREE) that these two works carry on other retail sites.