What are some good ways to free up disk space on OS X >= Yosemite? I am fed up with all these useless "Delete old pictures" "Delete your trash" BuzzFeed-tier articles so I am hoping for some qualified ideas here.

What I've done so far (and how much it deleted) is:


Monolingual (2.5gb) : deletes unused languages in applications.
I only kept Chinese and English. It can also delete classes for older architectures but that was only a few mb, languages is the main focus of that app. Many people say it destroys your applications but I've been using it for 5 months straight now and never experienced any problems
OmniDiskSweeper (countless gb) : like TreeSizeFree on Windows it lists directories by size. 
This way I for example found about 8gb of iTunes cache files that haven't been deleted, some xCode caches and simply big folders of other applications. If you "lower your standards" every session you will eventually end up even looking for folders smaller than 100mb but it's totally worth it as e.g. 6 of them already make half a gigabyte
CCleaner (5gb) : well-known on Windows but not too popular among mac users it took care of my cache files (basically Chrome (2.2gb) and Ableton(1.5gb) but it also deleted some system caches
OnyX (300mb) : it's rather fixing stuff than deleting but it also looks for some temporary files that were meant to  be - but never have been - deleted
deleting Garage Band (maybe 3gb) : as I am using nothing but Ableton for music production I deleted the app and most of its contents except for some fine drum samples


What else can i do? Having a 120gb MBA I have no choice but to locate every single file that's not completely necessary. It's just weird that, without installing applications, I see my drive getting more and more filled up with junk (I guess). Of course a system won't just magically lose "weight", but I can remember being at 45gb of free storage 10 months ago and my computer usage hasn't really changed since. (I am at 18gb now)

I might try CleanMyMac 3 at some point but I haven't heard about it too much yet.

EDIT: I will include your ideas in OP if it helped, but the best hint still gets a tick tho
More Suggestive answer supporting with everyone's answer as our main purpose is to delete unnecessary file and folder for iOS developer: 


DeviceSupport - You can keep the simualator which log's you may need in future. It's actually symbolicate crash logs for different device/iOS. 
Delete this folder after every few days interval

~/Library/Developer/Xcode/DerivedData
All your targets are kept in archived form in Archives folder. Before you decide to delete contents of this folder, here is a warning - if you want to be able to debug deployed versions of your App, you shouldn’t delete the archives 

~/Library/Developer/Xcode/Archives
iOS Device Support folder creates a subfolder with the device version as identifier when you attach the device. Most of the time it’s just old stuff. Keep the latest version and rest of them can be deleted (if you don’t have an app that runs on 5.1.1, there’s no reason to keep the 5.1.1 directory/directories). 

~/Library/Developer/Xcode/iOS DeviceSupport
Core Simulator folder is familiar for many Xcode users. It’s simulator’s territory; thats where it stores app data.   It’s obvious that you can toss the older version simulator folder/folders if you no longer support your apps for those versions. And, it’s safer to use ‘Reset content and Settings’ option from menu to delete all of your app data in a Simulator. 

~/Library/Developer/CoreSimulator 
Caches are always safe to delete since they will be recreated as necessary. This isn’t a directory, it’s a file of kind Xcode Project. Delete away! 

~/Library/Caches/com.apple.dt.Xcode
Additionally, Apple iOS device automatically syncs specific files and settings to your Mac every time they are connected to your Mac machine. To be on safer side, it’s wise to use Devices pane of iTunes preferences to delete older backups; you should be retaining your most recent back-ups off course.   

~/Library/Application Support/MobileSync/Backup


i got back about 40GB! For more help go to: http://ajithrnayak.com/post/95441624221/xcode-users-can-free-up-space-on-your-mac