If you're a developer and you find your HD space just seems to disappear without your putting any new files on your computer, take a look into what Xcode is doing. For every project you create Xcode will cache a lot of data for intellisense, autocompletion, and some other features. Whether it's a test project or a full on client project, Xcode will keep this data until you delete it. On my laptop I don't think I've deleted any of this data for about 8 months, which worked out to be roughly 16GB worth. This includes archived apps I send to clients for testing purposes, stuff that I consider to be a one off.