pump.io
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pump.io is an open source social stream server project built from lessons learned from implementing OStatus in StatusNet. pump.io as an interoperability approach is being superseded by the ActivityPub protocol.
See also, Evan explains pump.io on IRC: http://indiewebcamp.com/irc/2013-06-10#t1370892851
Evan also explains pump.io and federated social web on the TINAP podcast.
IndieWeb Compatibility
pump.io uses its own protocol for communicating between servers, based on ActivityStreams. There are no known examples of communication to/from indieweb sites.
Evan P has expressed interest in pump.io communicating via indieweb style comments.
Modern pump.io versions support microformats out-of-the-box. 
AJ Jordan (pump.io maintainer) has expressed interest in sending webmentions via a bridge.
Articles
Articles about pump.io
I really want pump.io to become one of those essential pieces of infrastructure for development. We have great, rock-solid servers like memcached or RabbitMQ that implement certain kinds of data routing. I think pump.io can be part of that toolset.