Review the code as it's been written, appoint a merger, select multiple people to review your work, discuss the best solutions with your peers, make sure all you code policies are applied and much more.

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.

If you've never had problems keeping your clock synced, you just haven't run enough machines yet. Once you start scaling beyond a certain point, it will become obvious that even elementary timekeeping is no small feat, even with help from tools like ntpd.

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

Colony graphs are a simple visualization of computer life forms. A process colony graph, or "ptree graph" for short, shows live processes and their parent and child relationships.

a fail2ban GUI powered by fail2rest

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

There are a couple groups of settings below. The first couple go in /etc/sysctl.conf or /etc/sysctl.d/filename.conf.

Each of these commands will run an ad hoc http static server in your current (or specified) directory, available at http://localhost:8000. Use this power wisely.