Giffy Cydia tweak allows you to view any GIF format images in your library in the native Photos app.
Check the video demo after the break.
You can also view a GIF files in any application that uses the native image picker, in SMS app for ex. All GIF images that are downloaded from Safari or manually added can be viewed after installing Giffy.
There are no options to configure, requires iOS 5 or higher.
Giffy jailbreak tweak is available in Cydia under the BigBoss repo for $0.99.