0# Windows 7 can be used in trial mode for approximately 120 days, if you don't have cash now (google it)
1# Student editions if you attend a university that has a deal with Microsoft.
2# Microsoft direct student discounts, requires an active .edu e-mail address
3# Discounted / free upgrades from Microsoft (Windows 7 to Windows 8 pro was $15 for 6 months) Windows 10 upgrade will be free apparently for one year from an existing Windows 7 or 8 installation.
4# OEM copies for Windows 8.1 are nice. This is a perpetual single machine installation. (As long as you only install it on one computer, you can install as many times as you want) Windows 7 OEM licenses are tied to the motherboard.