Romance/Erotica publisher Ellora's Cave offers their Naughty Nooners short stories free in ePub format at Barnes & Noble and Sony.
This search at B&N will bring them all up, you'll have to search for them individually by name at Sony using the
list at the publisher's site (no ePubs direct from them, for some reason).
Samhain Publishing (who, incidentally, have the best content warnings ever) also offers a few free e-books, usually as a time-limited promo. You can find them in the B&N store via
this search here; they don't seem to offer via Sony.
Both Ellora's Cave and Samhain's freebie offerings seem to be DRM-free via B&N, or at least have been on the past dozen or so I've downloaded and checked out of curiosity over the past couple of months to see if they'd keep it up, in case you were worried about compatibility issues with your chosen reader.
And there's the
free reads section at AllRomance eBooks. While not strictly all-erotic, they have a "heat level" rating which should help you weed out the tamer stuff and most of the offerings are available in multiple formats.
For GLBT tales, specialty bookstore
Rainbow eBooks has been doing promotions where they offer two selected stories free over the weekend. Not always ePub or erotic, but often multiformat and some form of romance, so you might luck out.
Finally, while it's not quite what you were looking for, there's some free (and DRM-free!) Harlequin and Mills & Boon romances with varying levels of explicitness that you might perhaps be interested in trying at their respective sites:
Try Harlequin,
Harlequin Celebrates 60 Years,
Everyone's Reading.
Hope this helps.