I've been a fan of Munin for many years, but as you get loads of servers and services with custom graphs, it takes a lot of time to navigate and build the views you want. So, I set out to build something which suited me more. The result is Illustrend.
It's an Erlang server, using Cassandra for storage. It has a simple HTTP GET interface for inputting metrics, along with a custom TCP/IP protocol. As it's written as a compliment or replacement for Munin, it supports most (if not all) of your existing Munin plugins as inputs.
On my quad-core i5 iMac, I manage around 50.000 updates per second, and I've so far tested it with over 650 million entries from my Nginx logs.
Getting time-series out of the system again is done with JSON, and there is a GWT frontend which lets you explore your data in real-time, and define your own graphs with drag-and-drop. Even with millions of tracked values, it only takes a couple of milliseconds to get data back out again.
If you're interested in trying this out for your own systems, please drop me a note.