Why do I get back more disk space when I restart my Macbook on Mountain Lion 10.8.3?
When the OS is low on memory it creates a file called a swap file, aka virtual memory on your hard drive, and in OS X this starts at some amount (I think 64 MB) then double as needed. So if you have 8 GB of RAM, you could end up with an 8 GB swap file. This is then deleted when you reboot.

You can read about it on http://en.wikipedia.org/wiki/Paging

Where the files are located and how to disable it: http://osxdaily.com/2010/10/08/mac-virtual-memory-swap/