If you have lots of time, you could go rummaging around your drives in Explorer (or the command-line if getting nothing else done in the next few years doesn't bother you) in an effort to reclaim space. Alternatively, you could do what the rest of us do and fire up WinDirStat or SequoiaView, which display the files and folders of your hard drives as coloured blocks of various sizes. With these visualisation tools, it's easy to identify where your space is going.
Then there's JAM's TreeSize, which is ridiculously fast at calculating the size of directories -- it puts Explorer and the other programs I've mentioned here to shame. It doesn't break down where your drive space is going as well as WinDirStat or SequoiaView, but if you're just trying to find the biggest culprits in terms of folders, then it'll sort you out fairly quickly.
These programs are great, but sometimes your space isn't being consumed by large files, but by many smaller ones that could be duplicates, or at least very similar. That's where an app such as Fast Duplicate File Finder comes in handy.
Most of my drives are taken up either with backups, videos or games, and I imagine most have a similar breakdown. Between the aforementioned programs, CCleaner and the low price of magnetic HDDs, I'm able to run a tight ship, but I'm interested to hear what you guys use.