I got sick of GTA IV's default car handling, and I disliked the one's I found on the internet, so I decided to make my own.
Basically, the cars now:
-turn sharper and more stable
-pull up faster
-have much more powerful brakes
-and you have better overall control
-and some other minor changes
To install on version 1.0, 1.01 and 1.02, just replace my handling.dat with yours in the \Program Files\Rockstar Games\GTA IV\common\data folder.
for patch 1.03, (this is not my solution) follow these instructions: http://gta-all.de/page/include.php?path=download&contentid=398, then replace the handling.dat
I hope you enjoy.
-more tiregrip, to prevent cars from flipping over, their center of gravity has been lowered
-handbrake power has been decreased
-car steering angle decreased, so finetuning corners is easier