If you are looking for a quick and easy magic bullet to significantly reduce your DB space used, there's only really one: use Page Compression on your (biggest/all) tables if you have not already done so. Yes it does have some real and potentially significant costs (addl. CPU overhead, primarily), but it's a one-shot fix that can have a big impact.

and of course, if you want that additional free space to be turned back to your disks, you will have to shrink (and defrag) the files some after that.

That said, the only other course is, as everyone has said, to get space back the old-fashioned way: delete/archive data. But that's real work to do it correctly so it needs to be a well thought out process.