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.