One of the best arcade racing games of all-time.
With more than 100 races, tons of cars, lots of tuning options, NFS Underground is incredibly fun and addictive. I've spent at least 50 hours playing the game on one of my old PCs. The graphics, today, may sound weak, but in 2003 they were awesome. The sound is great, and the soundtrack is one of the best of all-time in a game, of any genre.
The game plays well with keyboard or joystick, and even the drift stages are easy to learn (and easiest to master than today's drift events on games). And there were draft events, something I miss on some more recent games, like NFS Shift.
Finishing, Need for Speed Underground is one of the best arcade racing games of all time. If you never played, go check it out.