You can save hard disk space without deleting anything! Yes, it is possible thanks to hardlinks.
You can think a hardlink like a pointer to actual data in the disk. Multiple hardlinks pointing to the same data don't take redundant space on the disk. If your actual data is 1GB, then total size of all of your hardlinks is 1GB regardless of the number of hardlinks:
Sometimes you have to keep several copies of some files in various locations. May be you don't want to delete them or you are lazy to select proper ones to delete :) How about to converting them to hardlinks? Hardlinks look like actual files but don't take extra redundant space in the disk. So, only you need is to figure out which files are replicates of each others and convert them to hardlinks.
Duplicate File Detective makes a good job in finding duplicate files. Various comparison options are really useful to find what you want: