Graphite is a highly scalable real-time graphing system. As a user, you write an application that collects numeric time-series data that you are interested in graphing, and send it to Graphite's processing backend, carbon, which stores the data in Graphite's specialized database. The data can then be visualized through Graphite's web interfaces.
Used by Etsy.
Etsy also has StatsD (blog post) which is a daemon that increments/times can be sent to, which aggregates them and sends them to Graphite.