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 pump.io as an interoperability approach is being superseded by the ActivityPub protocol.
Evan has explained pump.io in #indieweb chat and on the TINAP podcast:
- 2013-08-04 This Is Not a Podcast: #9 - The Prince of Aberystwyth (archived from the original)
- 2025-06-05: Episode page still available on Tumblr with episode audio hosted on Internet Archive (OGG mono, OGG stereo)
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
Interview with Evan Prodromou on OpenSource.com:
"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."