Cleaning my Mac

The other day I spent like 4 hours removing unneeded stuff from my PC. It took me this long because I had several old files like unused VirtuaBox kernel extensions (I remember installing them when trying to learn some WebOS programming), numerous useless plugins all over the computer, other kinds of extensions, trash, etc. I googled each one of them to make sure I don't delete important things by mistake.
This is a list of things/places to check so in the future I don't have to google everything again. :)

1. First, EtreCheck is a tool I like it very much because is not offensive or intrusive as Onyx or similar tools, unlike CleanMyMac 2 it has no ads (CleanMyMac is crap). EtreCheck is free, relatively fast and open source.
   EtreCheck attempts to just report the facts, without making any judgements or recommendations. Debugging is not a task that can be automated. It needs humans, ...
   Just run the app and it will tell what extensions, plugins, etc is loaded in the system. These are good places to start to look for.
   
   
2. Login items Go to Preferences > Users & Groups
   Delete all unneeded items from from the list. Example, Dropbox, Google drive. All those things I don't use anymore. I only use OneDrive now :).
   
   Also read this: Stackexchange - What is the yellow warning sign in the login items tab....
   
   
3. Daemons and/or agents.
   Look for things like google talk, Adobe ancient stuff, etc. Legacy stuff (prior to 10.4)

   /Library/StartupItems
   /System/Library/StartupItems
   

   Items loaded at Mac starts up time. Run as the root user.

   /Library/LaunchDaemons
   /System/Library/LaunchDaemons
   

   Items loaded when any user logs in. Run as that user.

   /Library/LaunchAgents
   /System/Library/LaunchAgents
   

   Items loaded only when ${USER} user logs in. Run as that user.

   ~/Library/LaunchAgents
   

   Useful commands:

   $ launchctl list
   $ launchctl list | grep -v "com.apple"
   

4. Kernel extensions This places are quite sensitive. Make sure you don't delete unwanted things from here of the entire OS might break.
   

   /System/Library/Extensions
   /Library/Extensions
   ~/Library/Extensions
   

   Completely uninstall VirtualBox. Deletes kernel extensions, all VBox* binaries, Application Support and etc http://t.co/MQqSfyowjX

   — Ignacio Enriquez G (@nacho4d) January 10, 2015

   What is AppleKextExcludeList.kext? Now I know. Mavericks - to be locked down (in the future)? - OSx86 10.9 http://t.co/dlB5NuG0vu

   — Ignacio Enriquez G (@nacho4d) January 10, 2015

   Cnet - Increasing system stability by pruning the kernel extension folder
   
   
5. Internet plugins
   Make sure you have read this article:Unsupported third-party add-ons may cause Safari to unexpectedly quit or have performance issues and don't delete Apple stuff incorrectly.

   /Library/Internet Plug-Ins/
   /Library/Input Methods/
   /Library/InputManagers/
   /Library/ScriptingAdditions
   ~/Library/Internet Plug-Ins/
   ~/Library/Input Methods/
   ~/Library/InputManagers/
   ~/Library/ScriptingAdditions