Transport Tycoon Deluxe. I used to spend so much time on it that my dad made his own program to stop me going on it on certain days (ie. only Sunday was permitted).
Due to my ignorance, I didn't know about bypassing things like this. If only I had, I could have spent hours, and hours, and hours, and h...
I know at one point I got addicted to the games on a Sinclair card thingy (old pc thing). Got it out of the attic about a month ago, and it still works. Impossible to work out the controls which are annoyingly unaccessible whilst playing and not edittable. Still, when I was about 2, it was my first insight into gaming...Wow...
Others include:
Train Town (I hate this game now :/)
Starcraft - Not too long ago, but I remember having the demo version for about 4 years...Good times :-)