I just discovered this one...
Sometimes, your RAM fills while you are working and it is not a good time to reboot.
With three commands, you can clear out some RAM:
sudo sync
sudo sysctl -w vm.drop_caches=3
IMPORTANT!:
You must run the following command again, or information currently in RAM that you are using and need could be lost
sudo sync
After running all three of the above, you should see a marked increase in Available RAM.