We've played around with:

OpenTSDB: http://opentsdb.net/

StatsD: https://github.com/etsy/statsd/ (description here - http://codeascraft.etsy.com/2011/02/15/measure-anything-meas...)

OpenTSDB and StatsD seemed great for getting TS data in and producing nice dashboards, but they didn't seem to fit our needs for performing custom analytics on the data.

At the moment, we're leaning towards leveraging Cassandra based on our scalability requirements. Check out http://rubyscale.com/blog/2011/03/06/basic-time-series-with-... and http://www.datastax.com/dev/blog/advanced-time-series-with-c... to get an idea on how cassandra can help.