Development topic: http://gtaforums.com/topic/880608-

Introduction:
Jump and Fly is a modification for Grand Theft Auto III and Grand Theft Auto: Vice City utilizing the CLEO Library. It allows you to continue jumping while airborne, effectively allowing you to travel through the air. Type "JUMPANDFLY" to toggle this effect on or off.

Installation for GTA III:
* Download the mod for GTA III (click the Download Mod button).
* Download CLEO for GTA III at http://cleo.li/.
* From the CLEO download, copy III.CLEO.asi into GTA III's folder.
* If CLEO.asi already exists in the folder, you must delete that file.
* From this mod download, place the CLEO folder containing SEJump.cs into GTA III's folder.
This mod has been tested only on III v1.00 US.

Installation for Vice City:
* Download the mod for Vice City (click the Download Mod button).
* Download CLEO for Vice City at http://cleo.li/.
* From the CLEO download, copy VC.CLEO.asi into Vice City's folder.
* If CLEO.asi already exists in the folder, you must delete that file.
* From this mod download, place the CLEO folder containing SEJump.cs into Vice City's folder.
This mod has been tested only on VC v1.00 US.

Video: