I personally recommend some edits to the list:
Windows
Microsoft Security Essentials - this needs to be
removed. It's been exploited and virus-ridden so many times that it's not beneficial like it once was. Personally, I recommend
Avast - it's simply one of the best antiviruses, and for free too!
Mac
AppCleaner - Holy crap, this thing saves your life! Whenever you want to delete an app, instead of just deleting the app, open AppCleaner. It will remove the app's junk along with deleting the app itself, making it a REALLY handy app!
Caffeine - This app will prevent your Mac from locking. This is particularly useful whenever you're wanting to present something in class or you're trying to work on a project and your Mac keeps going to the lock screen!
NoSleep - This app prevents your Mac from going to sleep when you close your lid or waiting too long. This is really useful when you want to download stuff while you're sleeping and you want to close your lid!
Parallels ($80) - This nifty app allows you to run Windows while you're on your Mac, and seamlessly! It's surprisingly lag free as well. It costs a lot of money to set up, as you'll need a legal copy of Windows and a purchase of Parallels. If you're a college student, you can get Windows Server 2012 R2 (equivalent to Windows 8.1) for free through
Microsoft DreamSpark, and you can purchase
Parallels for $40.
Windows and Mac
Gimp - This is seriously my #1 go-to app for digital art. It's seriously like a free Photoshop. Love it, love it, love it!