Seeing many many grey cars out there sucks, and its because most peoples internet upload is pretty much shit. i was looking around at various tutorials for something completely different when i came across this page. It basically shows you how to create a file that trackmania interprets as a download link to the files that most people would just be uploading from their computer.

basically it makes your skin or custom item get downloaded very fast.

http://www.trackmania-carpark.com/faq.php its in that faq and many other places, but this tut explained it the best for me. i used dropbox.com but i'm sure there are other places you can use just as long as it is a direct download link.

Also, if you're looking for a place to store your skins and models once you've created a loc, Dropbox has a public folder that works pretty well. I'm currently using it to serve music into the game.

So should this file end in .loc, or do you need to leave the .txt extension on it?

yourcar.zip.loc

So I have the actual car skin in my public dropbox, where do I put the .loc file?

Quick and dirty explanation on how to use loc files: 
 
Lets say you have car skin named SkinName.
 
1) Upload your SkinName.zip to somewhere on the internet. (I use dropboxs public folder)
2) Copy the direct URL of that zip file. (example: http://dl.dropbox.com/u/usernumber/SkinName.zip)
3) Go to \MyDocuments\ManiaPlanet\Skins\Vehicles\CanyonCar (you should see all of your skins) and create a txt file
4) Paste the URL and save
5) Rename the txt file to SkinName.zip.loc
 
edit: Why would you do this?
Imagine you have just created the best skin ever. In order to other to see it, you'll need to send it to each player individually.
If your skin file is let's say 4 MB, and you connect to a GB server with 49 players, you will need to upload 49*4 = 196 MB ! (how fast is your upload ? :) )
With loc files the total size you will need to upload is less than 49 kb, and after that each player can download the skin simultaneously from the web host.

Awesome tutorial!

@mmzOne: Props to you, and to @SharkMan: for bringing this to my attention in the first place.

Great tip, thanks for posting it!

Where's the best place to store your skins/wav files? Dropbox?

I guess this is a good use for the webhost I barely use but keep paying for.

I have two custom gb cars I spent an unhealthy amount of time making (struggling with the alpha channel in Gimp2), created .loc files linked to zips in dropbox, and continue to be told I have a blank car. The first has a bunch of home-made stickers, and the second has just two giantbomb logos, and I have run out of ideas of what could be going wrong. The direct public link seems functional, and the cars work fine for me. Any thoughts?

@tehnuggies: i have no clue, i have the same problem on 1 of my cars. I wish i knew what was wrong with it, only thing i can possibly think of is that the thing is case-sensitive. really i have no idea.

seems like names of things are case-sensitive, i changed the name of a car skin and now people can see it, i bet spaces are also a no no. but to completely honest, i can't confirm this, other than this one instance.