The first tool in any modders belt is GTACars, the program which allows you to import new cars (which are actually just flat pictures) new textures, change the appearence of weapons (mayo gun from Burger City) and change in-game graphics.
GTACars - Jeff Matthews - registration is recommended.
The second tool is for the people that want to make thier own cities. It is called Junction25, and is the fastest way to make maps, although its strict top-down only view can lead to you making fences all over the place without realising!
Junction 25 - Jim Purbrick & Richard Field
Note: There is a J25 2Beta on the site as well. This has a few bugs, but an excellent 3D view once you get used to it.
Another useful map editor is called M1. This has a full 3D view and, although much slower to use to make the map, allows checking of flipped tiles to get corners right (waggles finger sternly at general map-making community, except Delfi and Pennywise) and also makes doing tunnels, bridges, sloped rooves and other high-art features possible!
gtamp.com/GTA1/m1win.zip - doesn't work on Windows XP - not the homepage
N.B: ABSOLUTE NECESSITY for doing railways!
To have a FULLY functioning map, with police routes, road blocks, ambulances, fire trucks, hospitals, police stations, fire stations AND proper railways you need a util called "GTACops" which only runs in DOS. So XP's COMMAND.COM can't hack it. Which is one of the many reasons I stopped GTA1 modding, in fact...
GTA Cops Download - NOT it's homepage
A tool which at least gives you a pleasent environment to do mission editing has been made by Delfi, long-time make of small but incredibly useful utilities for editing the GTA series. None other than he who made the TXD editor for GTA3 modders, in fact.
Delfi's Program Page
Junction25 has an excellent set of help files with it, although specialist tips will be posted later.
Now it gets really serious. The mission file is notoriously incompatible with human operating systems. It's line number basis and 2-part structure make it a total headache to work with. You'll often find yourself spending an evening re-numbering hundreds of lines just so you can fit in an extra DEAD_ARRESTED check.
The best place for tutes on this is the site by our resident Stilton, with some contributions from me, hosted by the infamous Gouranga site.
GTA1 Mission Guides - Stilton, Azz & Cerbera
Azz's site has accumulated a lot of benchmark tutorials - most by him! They cover all sorts of things, from making cars and respray hacking them to hex editing your map to add area names.
GTAzz GTA1 Tutorials
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If you know the actual proper home page of any editors then post them, I'll add them to the list. But, and let me make this absolutely clear, they have to be the ACTUAL HOME of the program - NOT just a site that hosts it.
Also any sites with original tutorials...generally go for the sites which are the source of the tutorial rather than the ones that just host it.
Edited by Sektor, 02 February 2008 - 06:21 AM.