InfluxDB – Open-source distributed time-series, events, and metrics database
I was actually looking at a bunch of open source time series databases and settled on kairosdb but this looks pretty nice.I think there is a hackernews rule someplace that a more interesting tech alternative shows up right when you decided to go with something else.For reference here is the list I created when researching these:http://opentsdb.net/overview.html 
Built on HBASEhttp://www.gocircuit.org/vena.html
Built on go + go'circult uses google's leveldbhttps://code.google.com/p/kairosdb/
A rewrite of opentsdb which can use Cassandrahttp://blueflood.io/
Built by Rackspace, decent but still seems a bit immaturehttp://graphite.wikidot.com/
Obligatory Graphite reference
(uses whisper, new backend called 'ceres' is being developed)https://github.com/agoragames/kairos (yet another 'kairos', alternative backends for graphite - SQL, redis, or mongo)Riak seems to be popular with SASS metric providers (hosted graphite, boundary).  There isn’t any code but there are a couple of talks that explain how and why they went with Riak:http://basho.com/hosted-graphite-uses-riak-to-store-all-cust...
http://boundary.com/blog/tag/tech-talks/http://vimeo.com/42902962