For me, It's ROBLOX. I remember discovering a game called Downhill Derby and playing it for hours on end with all kinds of other players dressed up as the old zombie. (green skin, brown pants, etc.) I can remember some of the cars and the environment as well. It was a sunset down a blocky/rocky brown hill, which had a row of about 9 cars you could enter and would respawn at random intervals if they were destroyed or were abandoned. They were extremely fast and were unstable as hell. They seemed to accelerate at random intervals when entered by a player. (unlike the old roblox vehicles where you had to equip a empty tool named Drive or something like that and hit keys like H or X or Y to drive to where the mouse pointed)
You couldn't control the cars. You just got in, waited for them to drive down the hill, and see what happened. Sometimes you'd veer off and crash, or you'd get lucky and make it to the end and hit a ramp. I even still remember the logo on it. Black ramp, red and white stripes on the edges/sides, and a yellow biker helmet logo on it with a black visor on a red circle.
EDIT: I actually found a vintage YT video on it! It's actually called Motorstorm:Downhill Extreme. Here's the link.
https://www.youtube.com/watch?v=HimeJ_NBMFY