OldNewExplorer is the Windows 8.1 (and Windows 8) shell extension / tweaker which can undo "improvements" to file browsing made in newer Windows version. Everything is optional for your liking.
– Restore devices / drives grouping
Everything is 'Devices and drives'. That's stupid! O-N-E can restore grouping which made sense in Win7/8.
– Restore libraries / remove folders
Remove all those folders which were unoptionally added into Windows 8.1 This PC folder and put the libraries back to their place, without registry hacking / breaking apps.
– Restore command bar / remove Ribbon
Because some people consider it an abomination.
– Restore details pane at bottom
Same reason; reclaim useful horizontal space.
– Improve styling
Get rid of some Win7 stuff here and there; improve styling with some built-in visual styles.
For themers / skinners, O-N-E can help to override a lot of hardcoded styling in Windows 8, maybe even Start screen!