There is also some not free other good program but the name just does not come to my mind. It has a nice graphical interface and maybe gives you a better overview. If I remember the name I will post the link too. There are two of them, that look very similar. But the one I mean is better known here. Maybe someone other knows the name. It shows the files in some kind of circle.
I found it. It's called DaisyDisk and there is a free trial version:
And I would recommend to search for an official way to uninstall apps before you just drag them in to trash. For most apps especially from the app store just trashing them is enough, maybe some leftovers stay in the library but they do not really take much space and don't slow down your Mac in most cases.
You could run the free EtreCheck after you deleted everything. It maybe shows you if unused helper tools are still anywhere.
Please don't use CleanMyMac (from the same website) or anything like that. It could help you scanning for things and maybe manually uninstall single apps with less leftovers. But the automatic cleanup and other scrips could really harm your Mac if it deletes something wrong.
But there are harmless App Cleaners in the App Store, where you can uninstall apps and it deletes most leftovers without being able to destroy anything. I think that one here is one of the best and for free:
I think you're remembering and referring to "Daisy Disk." http://www.daisydiskapp.com This is available on the Mac App store, too. It's a terrific app! In fact, I just installed and used it today on two of my computers to resolve an issue I was having with one of them displaying a lot less available disk space than the other, although both are configured very similarly and have the same docs, photos, movies, etc., in them. Daisy Disk showed me what I needed to see and I was able to look at the offending area, resolve my issue and delete the culprit, which was some kind of cache that had filled up way too much.
Something I use for periodically cleaning "house," so to speak, is OnyX, which is now available for El Capitan. It works nicely for doing a lot of the usual routine maintenance "housekeeping", and I've used it for years through various versions of OS X.
You can see what's on your HD by opening a Finder window and making your way through the folders. You can get above the top level of the HD by command-clicking on a window's title, scrolling down the the bottom of the list and selecting the computer name.
Is it unreasonable that your HD is full? What do you think is taking up the space?