In the era of "the cloud", it seems odd perhaps to be concerned about how much free disk space your computer has left. But there are still plenty of reasons why you might run out of disk space. In my case, my Mac laptop has a relatively small solid-state drive (SSD) and I'm one of those annoying people that decides he wants to use both Mac OS X and Windows, via Boot Camp.
So where the hell has all my disk space gone? It's not just the fact I've donated 20 GB to Windows. My Documents folder is only a few gig, as I save the majority of my work related content to a office cloud server. And my Applications folder isn't big enough to justify the lack of space.
This invaluable - free - app scans your hard drive and shows your usage in the form of a treemap. Essentially, a quick visual way of showing which files and file types are consuming space on a drive. You can home in on problem files just by hovering over them, or get an idea of whether you need to move your photo library somewhere else.
As you can see above, I've got a big file taking up quite a large amount of space - though clicking on it shows it's actually my Mac's sleep image. The last time I ran this, it showed me a large archive of old emails that I didn't need anymore too.
Disk Inventory X works on Mac and Windows partitions, though it's a Mac-only app, so it is helpful if you're trying to diagnose disk issues on a Boot Camp partitions. However, there's the similar WinDirStat if you're a full-time Windows user.