Below are a few areas of macOS that might be eating up your storage.
REMEMBER: Delete files at your own risk and always have a backup. We use CrashPlan.
Migrated iPhoto Library
If you use Apple's Photos then look for an old iPhotos Library.migratedphotolibrary in your Pictures folder. Wait to delete this library until you are very sure that every photo migrated to the new Photos app. I've read online that this folder is not actually taking up space. However, when I deleted this 80GB file it freed up 56GB from my Mac.
iTunes Device Backups
Look in your iTunes > Preferences > Devices and delete any iPod, iPhone or iPad backups that you no longer need. I currently backup to iCloud so I don't have any devices listed in the screenshot below.
iOS Apps On Your Mac
If you backup your devices (and apps) to iCloud, but you once backed up locally with iTunes, then you may have a Mobile Applications folder in Music > iTunes that you can delete. This folder on my Mac was 17GB and hadn't been modified in over a year.
Microsoft Outlook 2011 Identities
If you've used Microsoft Office 2011 for any period of time then you've probably had to rebuild the database and you might not have deleted the copies it made. Look in Documents > Microsoft User Data > Office 2011 Identities for old copies you can delete.