Windows 8 & 10

Classic Shell

Hate the Modern UI of Windows 8. Keep Windows 8 and get back the missing START menu.
http://www.classicshell.net/
Project page on Sourceforge.
Note: Classic Shell has moved to it's own website so the information on sourceforge.net may not be the latest.
http://sourceforge.net/projects/classicshell/
Download:
http://www.classicshell.net/downloads/

Advanced Tokens Manager

Microsoft imposes a limit on the number of times a copy of windows can be activated online or by phone. Advanced Tokens Manager is able to make a full backup of all activation data stored on your computer and allows the data to be restored without damage or modification of the activation system.
http://joshcellsoftwares.com/products/advancedtokensmanager/

StartOnDesktop

StartOnDesktop will enable the Desktop to be displayed at every logon instead of the Metro Start Screen of Windows 8.
http://joshcellsoftwares.com/products/startondesktop/