Everything benefits from a good spring clean.....
The first time I ran a duplicate file finder it took half an hour to run, and then hours (literally) to dig through the results and confirm which files to remove. This painful experience made me realise that doing it regularly is a much less unpleasant process so now I set myself a reminder to run it once a month. This keeps my computer tidier, my hard drive sleeker and speeds things up.
The worst offenders are photos, videos and music. A good duplicate file finder will also dig out 'similar' files so that you can eliminate the 19 almost identical pictures of your dog/child/fireworks display and just keep the best one.
Firstly - back everything up to a USB key 'just in case'. You never know if you'll wake up tomorrow and suddenly want those 45 almost identical pictures of the sunset.
I use Gemini 2 Duplicate File Finder by MacPaw on my MacBook. It gets consistently great reviews as the best all rounder and has served me well over the past few years. It's not free, but all good software has a price so that developers can continue to make sure it keeps performing well despite those continuous updates and upgrades that both Windows and Mac constantly demand.
If photos (on Mac) are your primary concern, then PhotoSweeper X is the one for you - it makes tidying up thousands of images a lot quicker than other duplicate tools as it specialises in photos. You can easily compare images side by side before choosing which to remove.
Most of these tools are either Mac or Windows specific given how different their operating systems are.
Windows has a great built in tool called Disk Cleanup which already frees up considerable space. But to find all those duplicate files, try Duplicate Cleaner Pro or Wise Duplicate Finder. Easy Duplicate File Finder works on Windows and Mac which is handy if you use both systems for work and home and would prefer to only learn how to use one app.
Make sure you have some time before running any of these programs as although they mostly come with a 'delete instantly' option, you should take the time to browse through the suggestions before letting it wipe away your files. I find that I sometimes want to keep 2 copies of the same file (in different folders for example). Most software can be configured to eliminate specific folders from the search so the more you use them, the better you can configure them to suit your filing system.