Are there hidden files taking up 10GB on my hard disk? I was cleaning up my hard drive this morning, 128GB ssd in a macbook pro, and there seems to be about 10GB of data that isn't appearing in the exposed file system.

I'm using the GUI interface with cmd+click -> get info, here's the breakdown:
Hard Drive: 98.8GB on disk

then in the Hard Drive Directory:
System: 6.5GB
Applications: 23.7GB
User:47.5GB
Library: 11GB
opt:174MB
User Information: 4 KB

that adds up to ~89GB. Where are the missing 10GBs? I'm starting to regret not springing for the bigger hard drive, though I guess it would just postpone the issue.

Are hidden files hidden from the file-by-file get info panel?
There are easily 10Gb of files hidden from the Finder.

Get Info on a disk and Get Info on a folder work differently. On a disk, it simply asks the disk resource for a count of the used blocks and thus gets an answer almost immediately. This value accurately reflects how much is stored on the drive in question.

Get Info on a folder digs through that folder and adds up the size of each file that you are able to read. You cannot, for example, know how much another user has in their Mail folder by running Get Info on the User folder.

Also, there is a difference between the file size and the space-on-disk size. Storing a single character in a file will consume 4kb of disk space, the rest is wasted. One million one-byte files will therefore use 4Gb of space on disk.

Files hidden from the Finder include almost the entire Unix subsystem, many low-level config files, virtual memory, sleep images, and the system caches. Don't even think about deleting them - your computer will not boot, and the caches will simply get refilled.

On the plus side, the Mac OS takes care of itself quite well. You the user, have absolutely no need to concern yourself with the inner workings, nor do you need 3rd-party utilities to do any maintenance (most of them don't do anything effective except reduce your bank balance).

A very easy way to reclaim a lot of space is get an external and move your iPhoto and iTunes libraries there.