I now have yet another excuse for not posting here much: OpenTTD. Its aim is to be an open source clone of Transport Tycoon Deluxe, although right now you need the original game for the data files for it to work.
And unlike the original Transport Tycoon series, this is cross-platform, with official builds for Windows, Linux and Mac OS X, and there’s even a PocketPC build lying around. Hari and I have both been playing it a lot, including as a network game against each other, though you can play online as well should you wish. One thing to note as that you’ll need to open port 3979 on your firewall, especially if you use a Mac.
I used to play Transport Tycoon almost incessantly when I was younger and didn’t have internet access; this take me back to those halcyon days. It’s surprising how much I remember about it too, bearing in mind it’s probably been 7 years or so since I last played it.