I suppose it makes sense that since the the main author behind the PSR-4 Autoloader recommendation is also the lead on the Aura project, that Aura would have the first PSR-4 implementation. You can download it here

and you can find the README here (or in the download, of course):

I expect Composer will have support for PSR-4 soon as well.

Via http://auraphp.com/blog/2014/01/03/first-psr-4-autoloader/.

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