My favourite is UltraISO (not free) and ImgBurn (free)
Misread question, my badImgBurn is a burner, not a mounter.
With Win 8.1 (and Win 10), double-clicking on an ISO automatically mounts it, no need for 3rd party software. Right-click on mounted drive and select Eject to unmount.If you have Windows 8.1 you can usually just double-click on an ISO and voila the contents of the ISO is shown in Windows Explorer (if the ISO format is not associated with any other program) and that might remove the need to 'mount' the ISO if you just need to look at the contents of the ISO file.
But if you do need to mount an ISO I remember using Daemon Tools Lite which was free - might want to check that out.
If you have Windows 8.1 you can usually just double-click on an ISO and voila the contents of the ISO is shown in Windows Explorer
But if you do need to mount an ISO