Lepton is an open-sourced Dropbox technology for the lossless compression of JPEGs.
The program can compress JPG files by 20% or more, exporting the results in a LEP format (picture.jpg > picture.lep). Point the program at LEP files later and it restores the original JPG file, byte for byte.
The download is a single executable file which runs from the command line. The simplest option is to pass Lepton the file you'd like to process, and it's converted accordingly (JPGs are compressed to LEPs, LEPs are restored to JPGs).
The program doesn't make much effort to check its input. You might expect to run it like this...
...but that gives you an error. You must include the ".exe", like this:
If you're including a file path with a space, be sure to surround it with quotes, or the program may crash.
lepton.exe "c:\my pics\mypic.jpg"
If that sounds like too much hassle, we've put together LeptonGUI, a very simple front end which accepts files by drag-and-drop. This doesn't have any other functionality, though, so if you're happy with command line testing then the main lepton.exe is all you need.