Have you checked out OpenTSDB yet? It's pretty nifty.
OpenTSDB is a time-series database built on top of the venerable hbase. It allows you to aggregate and crunch many thousands of time-series metrics and digest them into useful statistics and graphs.
But the best part is the tagging system that allows you to build dynamic and useful graphs on the fly. With every metric you send you simply attach arbitrary tags "datacenter=ec2 cluster=production05 branch=master". Later on you can bring these up to compare minute differences between systems.
This kind of monitoring blows "enterprise" solutions like Zabbix and Nagios out of the water. There's no way you could fit this kind of data into either rrdtool or whatever the heck Zabbix uses to store it (MYSQL??!?!). It's also an "agentless" solution, which makes it well suited for the cloud.
TcollectorNow you can get realtime metrics on how your varnish web accelerator is doing. I wrote a tcollector plugin to slurp counters from varnishstat and send them to TSDB.
There's a pull request up to merge the collector into the tcollector repo, but in the meantime you can find the varnish collector script here.