Boxee, the "social" media center software for Mac OS X, Ubuntu, and Apple TV (with Windows on the way) has added ABC shows to its streaming line-up "just in time for the Lost season premiere". It only works on OS X and
Ubuntu at the moment — but it'll be added to the other versions soon enough.
Update: Clarification from the Boxeers:
I wanted to give you a quick update that boxee is available in a public Alpha for Mac and Ubuntu from boxee.tv. (go there, sign up, download immediately) boxee is available on private Alpha for Windows (go there, sign up, twiddle your thumbs waiting for an invite for a week or two, then download). boxee with ABC is available for the Mac version only at this point due to the Move Networks plug-in they use to deliver content (not available for Linux, and will take some more tweaking for Windows).