I know the GTR demo has been knocking around for a few months now, but I've just discovered you can play a wet weather version of the same demo!
if you look in the directory you installed the demo, there should be a file called player.plr. open this in notepad and find the line...
PRACT Weather = "2" // random/sunny/ etc.
change this to...
PRACT Weather = "5" // random/sunny/ etc.
start up the game and drive as usual, and you should be in the middle of a belgian downpour. The real men will drive this without Traction Control or ABS
I only just found out about this one so i'll give it a try when i get home tonight