Ask HN: What tools and practises you use for capacity planning / metrics?

I'm interested to find out what tools (if any) people are using for capacity planning and metrics on growth.

(By growth I'm talking about Database, Disk, Performance and of course number of users etc...)

Are people just using spreadsheets with input from monitoring sources, or are there some fancier tools out there? (F/OSS please!)

Background:

I work for a web app company with 300~ servers and we need to be smarter with our capacity planning, some of our larger apps don't currently scale well horizontally and I'm interested to know when and where to target our efforts (i.e. redevelop parts of the apps vs just throwing money at hardware / virtual hardware upgrades).