RSS 2.0 in JSON by Adam Kalsey. Kalsey assigns RSS 3.0 and joins a line of other RSS 3.0s, some humorous, some not.
It is important at this stage to note that there have already been far more variations of RSS 2.0 than of RSS 3.0, since RSS 2.0 is vaguely defined and inconsistencies or cases of underspecification are solved by checking with the whim of Winer. This whim, as is typical of one, changes periodically. Oh, and you’re not allowed to codify these whims and publish them as a newer spec, or version them and specify that version in your feed. That would be easy and help tools parsing RSS and we can’t have that.