I noticed that my 120GB SSD boot drive was slowly filling up, beyond what I could account for. WinDirStats was showing a growing "Unknown" block of space, so I finally had the idea to run WinDirStats as admin (duh), and I found NINE GIGABYTES of tiny JPEG thumbnails in "C:\Windows\ServiceProfiles\NetworkService\AppData\Local\Microsoft\Media Player\Art Cache\LocalMLS" - I don't even use Windows Media Player, but it appears that any audio or video file I've played with any player has a thumbnail in here. Podcasts I listen to had hundreds of duplicate thumbnails, one for each episode. I looked around on-line, and it turns out that the Windows 7 Media Player Network Sharing Service is to blame. It stores thumbnails as hidden files in this admin-only directory for everything, for the purpose of network sharing, and it doesn't clean up after itself, even after the media files themselves are deleted. I looked around on-line, and found that this is a common, known issue with Windows 7, to the point where some people are reporting that it's using tens of gigabytes of space (one person in the linked discussion on MicroSoft's website said he found 70GB of thumbnails). I've disabled the service to prevent this from happening again on my system (and deleted the files), but the problem persists for anyone who uses Windows 7 and isn't aware of this problem (I don't know if it also affects Windows Vista). Unfortunately, not only does Windows not clean up after itself here (nor does the Windows built-in drive cleaner app), but CCleaner doesn't currently look in this directory to clean out old stuff.
Given that most of the files CCleaner currently erases are things that Windows or the responsible apps mostly kinda-sorta manage and clean up themselves, adding this single feature to CCleaner could make a bigger impact on users' free space than all of the other features in CCleaner combined.
If you do add this feature, you might want to include a pop-up warning that it could take awhile. Since anyone who has been using Windows 7 for a yar or more is likely to have tens of thousands of files in this directory, scanning and deleting them all could take several minutes the first time the routine is run. Even on my fast SSD, it took several minutes to delete 9GB of ~50KB files. It'll take an order of magnitude longer on a hard disc drive.