for Xcode 8:
What I do is run sudo du -khd 1 in the Terminal to see my file system's storage amounts for each folder in simple text, then drill up/down into where the huge GB are hiding using the cd command.
Ultimately you'll find the Users//Library/Developer/CoreSimulator/Devices folder where you can have little concern about deleting all those "devices" using iOS versions you no longer need. It's also safe to just delete them all, but keep in mind you'll lose data that's written to the device like sqlite files you may want to use as a backup version.
I once saved over 50GB doing this since I did so much testing on older iOS versions.
That directory is part of your user data and you can delete any user data without affecting Xcode seriously. You can delete the whole CoreSimulator/ directory. Xcode will recreate fresh instances there for you when you do your next simulator run. If you can afford losing any previous simulator data of your apps this is the easy way to get space.
Update: A related useful app is "DevCleaner for Xcode" https://apps.apple.com/app/devcleaner-for-xcode/id1388020431
If you happen to be an iOS developer:
Check how many simulators that you have downloaded as they take up a lot of space:
Go to: ~/Library/Developer/Xcode/iOS DeviceSupport
Also delete old archived apps:
Go to: ~/Library/Developer/Xcode/Archives
I cleared 100GB doing this.
Try to run xcrun simctl delete unavailable in your terminal.
Original answer: Xcode - free to clear devices folder?
Source: Stackoverflow.com