Sysdig is open source, system-level exploration: capture system state and activity from a running Linux instance, then save, filter and analyze. Think of it as strace + tcpdump + lsof + awesome sauce.
With a little Lua cherry on top.

Short linux sysadmin screencasts containing tutorials, tips and tricks. Great for both novice and experienced Sysdmins.

A curated list of amazingly awesome open source sysadmin resources inspired by Awesome PHP.