Depends on what you mean by Open Source, the Onyx BOOX family of readers use android as their os, the Nook uses Android too with a Barnes & Noble look over the top of it. The pocketbook readers I think use Linux as do the Bookeen Cybook ones. All do epub with and without DRM and some do the mobi format without drm. Of course all of these are extremely costly because they don't sell direct to the US market because they can't compete with Amazon's loss on device make up in sells strategy. I've seen them on the Amazon UK site.

If you wanted to shop around hoping for better ebook prices (not likely thanks to publisher price setting) then you can use Apprentice Alf's Tools to break the drm (if legal in your country) and put the book you've bought on the device of your choice.

There are also less than reputable sites that put books up such as tuebl, but they claim to have some support and a DRM takedown policy as well as links to sites to legally obtain the item, I haven't ventured too far into their rabbit hole though.

My experience was I've had the PRS-750, PRS-300 and PRS-350 from Sony, I upgraded to the Kindle Touch when it came out because you couldn't beat the rock bottom price, sadly this is the strategy Amazon used to take something like 70% of the reader market in the US and if Sony can't compete with that what luck will Kobo or anyone else have? (I do have a Kobo Touch as well for books I want in epub.)