PNGOUT is a nice free software for lossless image size reduction. It comes in 3 forms:
1) The basic version of PNGOUT is a command line tool and does not comes with a UI.
3) PNGOUT is also available as a plugin for IrfanView. In that case, you get a nice dialog box to adjust settings. But compression is not guaranteed to be lossless with IrfanView.
It is really simple to use PNGOUT command line version. Just go to Windows Run box (in Start Menu) or command prompt, type PNGOUT with input file name and output file name, and PNGOUT will automatically do a lossless optimization of the input image.
An interesting feature of PNGOUT is that it can work on images that are present in your Windows clipboard as well. In that case, you do not need to give the input file name, and PNGOUT will understand that you want to pick the image from your clipboard.
PNGOUT provides many options to tweak the conversion. You can provide color type, filter type, color bit depth, strategy, and many more. You can check all the options here.
If you are looking for a no frills lossless image size reduction software, PNGOUT is for you. If you want to do a lossless image size reduction online, try Smush.It.
Download using links at the end.