When I first got an Xbox 360, I wanted to play EA's NHL 09. But of course, NHL 10 was on its way only a few months later, and I couldn't justify buying 09 just to have it immediately outdated. So instead, I downloaded the free NHL 09 demo from Xbox Live and played it nearly every day until NHL 10 arrived. I became intimately familiar with the 5-minute game sessions between the predetermined matchup of Detroit Red Wings and Pittsburgh Penguins.
For people without tons of disposable income, demos can be incredibly important. Are there any game demos from your childhood (or later) that you've got a sentimental connection to?