Based on resident evil 4, this modification makes all the ladders climbable by pressing a simple button. I created a ladder climb anim to make this possible ingame. This version shows only how this mod works and it isn't possible to load your saved game unless you have a bit experience in coding and know how to take the external script and implement it in your .scm.

Simply add ladder.ifp anim file to gta3.img and replace any main.scm and scripts.img.

You can find more information in the official topic mod.
http://www.gtaforums.com/index.php?showtopic=294235