Auto3G extends your battery life by automatically turning off the 3G radio (leaving on the 2G radio) when you lock your iPhone and automatically turning on the 3G radio when you unlock your iPhone. This way you always have the speed of 3G when you need it without the battery loss.
As explained by Apple, this results in up to 100% greater battery life: "7 hours of talk time on 3G, 14 hours of talk time on 2G." (apple.com/batteries/iphone.html
Auto3G can be purchased from the Cydia Store for $5.99