http://63.236.94.8/rockstar/classics/index.php
As part of their "classics" lineup, you can download the first GTA free. It looks like their servers are crapping out though as I've been trying to get the download going for about 20 minutes with no luck. I'll keep trying though. I remember playing the demo all the time back in... hmmm... that'd probably be my freshman year of high school. Ahhhh, smell that? That's nostalgia.
By the way, the file is 328 MB so be warned.