<p>Stack Exchange themselves use <a href="http://clearbits.net" rel="nofollow">ClearBits</a> (<a href="http://legaltorrents.com" rel="nofollow">LegalTorrents</a>) for their data dumps. But I think they charge for uploads. And they require stuff to be under a free* license.</p>

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

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<p>The above is no longer true. ClearBits have closed down, and <a href="http://meta.stackexchange.com/a/213990/147191">the SE data dumps are now hosted on Archive.org</a>.</p>