Macbook Pro i7, Hard Disk space filling up before my eyes (Mavericks)
gvb found the application that kept filling up his drive by running:

sudo fs_usage -f filesys

(at my suggestion) in a Terminal. This command allows you to monitor file system usage, such as reads and writes by different processes. 

In his case, "Avid App Man Helper" was filling up the drive continuously. Once this application was removed, his hard drive space was freed and no longer being continuously filled by this process.