Synei Disk Cleaner is a straightforward tool which aims to help you find and delete surplus files which might be cluttering your hard drive.
To find out how much space it might be able to free up, launch Disk Cleaner, make sure "Analyze Only" is checked, and click "Clean". This will scan your system for junk files and report on what it finds, but won't remove anything yet. (The idea of a "Clean" button which doesn't necessarily clean anything seems like a very bad move, to us, but that's the way it works.)
Disk Cleaner checks all the usual areas for files it can delete: browser leftovers, system files and assorted junk associated with many popular applications. And it seems to be very effective, too, with the program even outdoing CCleaner on our test PC (Disk Cleaner: 3.6GB to free up, CCleaner: 2.9GB).
What is it actually going to delete, though? The default report won't necessarily tell you much, for example just identifying around 500MB of leftovers associated with "Apple Products". Double-click this, though, and you'll see the various files it thinks are surplus to requirements, giving you a chance to vet what the program is doing.
Once you've decided you're happy, clearing the "Analyze Only" checkbox and clicking "Clean" will actually clean your hard drive, this time.
If you need more cleaning power, then you can opt to run "Full" or "Extreme" scans, each of which will normally also identify further files you can delete, but also have a greater risk of wiping something important.
And the "Tools" tab offers other useful options, including a "Deep Cleaner" which can identify files you can delete by their extension (*.old, *.tmp and so on).
Put it all together and this seems a very capable drive maintenance tool, but be careful - it only takes a single key file deleted in error to trash your PC. Check everything Disk Cleaner wants to do before you clean your system for real. And don't ever use it unless you have a full system backup to hand.
Synei Disk Cleaner generally finds a lot of leftover files to delete, but as with all similar tools, you need to treat it with caution. Only run the program if you've a full system backup to hand