iPhone’s address book is certainly not the best on the mobile phones. While competitors like Nokia, HTC and Samsung have a dynamic and multi-functional address book with features like searching contacts as you dial etc (this is one feature I enjoyed in my old Windows Mobile O2 Mini).
ABFix via Cydia brings dynamic index in your contact book. What it does is it smartly groups the least used letters by you together so that you can switch to another section as quick as possible. For e.g.|, you have less contacts from the letter X, Y, Z. So it will combine these three so that your search is made faster when you type. I feel this works better for non english languages, but this is at-least some smart addon to the iPhone contacts organizer!
However, this can also get quite irritating when you want to search the contacts from that particular word. But remember that ABFix is not an app, its an addon package to your address book and doesn’t have an icon or toggle.
Installs via Cydia and is available via RipDev Repository/Source.