OpenTTD is an Open Source simulation game based upon Transport Tycoon Deluxe.
This game was written by Chris Sawyer and mimics the original game as closely as possible while extending it with many new features. The list of enhancements and features compared to the original Tycoon Deluxe is too long to even begin with here.
OpenTTD is licensed under the GNU General Public License. For more information, see the file COPYING included with every release and source download of the game.
For OpenTTD you can use the original Transport Tycoon Deluxe data files from your CD. However, there are also the free alternatives that are linked to from http://www.openttd.org/en/download-stable. Windows XP users will need SP3 installed.