I just want to delete files in order to free up space on my hard drive. When I used CCleaner for the Windows it showed me a lot of things that I could delete like log files.
Other than your own user files, there's not a lot of space to be saved by deleting files in a basic Mac OS X installation. There's not a lot of "fat" that can be trimmed. You could use Monolingual to remove unwanted languages and unused architectures, like the PPC architectures, but that may only save you 1 or 2 GB.
Moving less-used user files to an external drive is a less risky option.
Onyx, Safari and iTunes are 3 completely different apps with completely different purposes. Learn what they do before suggesting something that makes no sense. Safari and iTunes have nothing whatsoever to do with this thread.