When I was a kid I bloody loved Unirally. It's beyond me why no-one's made a sequel or, at the very least, a game very similar to Unirally. Action Henk is in development and it may be the closest thing to it. Imagine Unirally crossed with Super Meat Boy crossed with Trials -- that's Action Henk.
Action Henk is the debut game from newly formed Dutch studio RageSquid and it's one of those games that you play and it instantly feels good. I love the core mechanics of it -- skid down hills, maintain momentum, gain speed. It's just a great little game.
You can actually play the first three levels of the game, which is still in alpha, here. I totally recommend that you do.