Stack Exchange themselves use [ClearBits](http://clearbits.net) ([LegalTorrents](http://legaltorrents.com)) for their data dumps. But I think they charge for uploads. And they require stuff to be under a free* license.

<sub>* Free in the Free Software sense, or one of the non-Free Creative Commons licenses, such as **ND** or **NC**. I'm not sure exactly what their requirements are, but it has to allow unpaid redistribution, anyway.</sub>

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The above is no longer true. ClearBits have closed down, and [the SE data dumps are now hosted on Archive.org](http://meta.stackexchange.com/a/213990/147191).