OS X :: Hard Disk Filling Up - Invisible Files?

The free space on my MacBook is disappearing fast but I don't know why. I am hoping someone can tell me where to start solving this problem. It must be invisible files but what can I do about it? This started after I restored from a Time Machine backup to a new hard disk after the hard disk failed (another story). The Time Machine backup was about a week old (isn't it always the case?) so I don't think it is likely that I could have restored anything that might have had anything to do with the hard disk failure. Everything seemed to be working fine, having spent hours resorting emails and trying to reconstruct files I had lost from the week I didn't back up. During that week I had bought an iPhone. When I connected it to the restored computer it deleted all the apps I had installed. Not a big problem, as I was able to track them down from an iTunes receipt and reinstall them. I then attached my iPod and the computer had to replace all the files with new ones. As there were a lot of songs and photos I left it to do this and went to bed. Next day the iPod now had newly replaced files and seems fine. But there is a warning that the startup disk is full. No new files had been put on the computer, which had about 9.5 Gb free space. I deleted some files so I could do some things with the computer to find out what was happening. The space thus freed soon disappeared. I then deleted Parallels Desktop so that I could free up more space. I now had 29.67 Gb free space. I tried searching for large files but nothing suspicious seems to detectable. By the end of the day the free space had gone down to 24.39 Gb. It now stands at 23.22.