There are plenty of free compression programs, 7zip among them, Google for more.
That said, it depends on which files you want to compress, these days there are not a lot of files that can be compressed by much.
The typical files that use a lot of space are, music, photos and videos. As these files are already compressed, there is little or no gain by further compression.
Best solution is to buy a new drive as they are very cheap now.