Auto3G, Extends your iPhone’s Battery Life by Automatically Turning Off 3G

Auto3G is a simple tweak in Cydia that increases your iPhone's battery life by turning off 3G raido (leaving 2G radio on) when you don't need it automatically. In brief, it turns off your 3G radio when your iPhone in standby, turns on when you unlock it.

This tweak is really useful in saving battery life as Apple explains "iPhone 4 offers up to 7 hours of talk time on 3G, 14 hours of talk time on 2G, 6 hours of Internet use on 3G" so this tweak may doubles your battery life. Auto3G is available in Cydia for $5.00 via Big Boss repo.