To elaborate, Cydia is an app store/package manager for jailbroken iOS devices. The US Copyright office and the Library of Congress have confirmed that jailbreaking your iPhone is not a violation of the DMCA, and it is very easy.

Evasi0n7[1] runs as an application on your PC or Mac with your device connected, jailbreaks the device and installs Cydia. It does not affect anything in the regular Apple App Store, it merely lets you install additional programs including tweaks that customize the interface. Perhaps interesting, f.lux is featured on the front page of Cydia, so you don't even have to search for it.

[1] http://evasi0n.com/