I just recently spent a whole day trying to find my copy of Rollercoaster Tycoon 3. Couldn't find it, then I thought I might have purchased it from Steam or something, but I hadn't.
I wanted to play a Tycoon game soooo bad, and I wasn't sure why. I searched and searched for something cheap but the only thing that caught my interest was Rollercoaster Tycoon 3 and Zoo Tycoon 2. I couldn't find Zoo Tycoon 2 (or even the first one) on steam or Direct2Drive, so I just broke down and got Rollercoaster Tycoon 3 on Steam.
Anywho, the only ones I have ever played were,
Rollercoaster Tycoon 3
Dino Park Tycoon
If you guys think that Zoo Tycoon (1 or 2) is worth it, I'll pick it up sometime.