We use Riak at Dropcam to store H.264 video streaming from cameras running 24/7 which needs to be available for playback on the website and mobile apps. We needed a block store that could scale to handle petabytes, tolerate disk and node failures, and provide always-on write- and read-ability.

Riak fits the bill very well, though we did need to make a few mods to workaround certain problems. But it's Erlang/OTP, fairly small code-wise, and easy to modify.