It's about time we got a thread together which helpfully lists all the resources we have for modders. I'll be expanding this thread as I get the time to do so. If there's anything which needs to be added, just reply here and I'll add it to the list.
Parameters for Structure.xml - a work in progress list of parameters used in vehicles' Structure.xml
Flummery by Errol - All-in-one tool for setting up cars, levels, etc. for C:R. Currently work in progress.
C:R Tools for 3DSMax by Trent - All-in-one script for creating and configuring mods in 3D Studio Max and exporting directly to the game.
LOLDecoder by Trent - A tool based on UNLUAC's source code for decompiling LOL script files. It doesn't handle all the files correctly but works perfectly for car & level configuration files.
GibbedTools modified by Trent - A tool for converting textures to and from C:R's TDX format, based on the source code of GibbedTools for Duals Of The Planeswalkers and modified to work with C:R.
MINGE by Trent - The CarMODgeddon Mod Installer
Historic Tools (no longer necessary/applicable)