you would be looking at the cowon S9, which is totaly flash based OS, and they 'want' you to make custom UCIs for it, there is a fan site for the S9 which has them already. If you meant "open source" like that
Yeah, the Cowon J3 looks real interesting. I wish the S9 had expandable memory capabilities.
Since I only bought my S9 last year, I'll simply wait for something to come along with higher internal memory capacity. 128GB flash memory would be nice, but probably at least another 5 years away.