OpenTTD is a clone of the Microprose game "Transport Tycoon Deluxe", a popular game originally written by Chris Sawyer. It attempts to mimic the original game as closely as possible while extending it with new features.
OpenTTD is licensed under the GNU General Public License version 2.0. For more information, see the file ´COPYING´ included with every release and source download of the game.

Significant enhancements from the original game include for example
bigger maps (up to 64 times in size)
an AI that is actually worthy of its name
stable multiplayer mode for up to 8 players on LAN or Internet
dedicated server mode and an ingame console for administration
new pathfinding algorithms that makes vehicles go where you want them to (usually)
autorail build tool, improved terraforming
crossing tunnels
canals, shiplifts
larger, non-uniform stations and the ability to join them together (of same width or length)
real checkpoints instead of dummy one-tile stations
mammoth and multiheaded trains
much more

Lots of effort has been put into giving OpenTTD a better usability. These include
convert rail tool (to rail, monorail, and maglev)
build stations with drag&drop
support of drag&drop for almost all tools (demolition, road/rail building/removing, scenario editor...)
sorting of most lists based on various criteria (vehicle, station, town, industries, etc.)
mouse wheel can be used to scroll in menus and to zoom in/out.
autoscroll (to left/right) when the mouse is near the edge of the screen
build in paused mode
sell whole train by dragging it to the dynamite trashcan
cost estimation with the ´shift´ key
patch options configuration window, change settings from within the game
"debtmax" faster loan management with CTRL key

Graphical/interface features
screenshots can be in BMP, PNG or PCX format (select in game options menu)
more currencies (including Euro introduction in 2002)
extra viewports to view more parts of the gamefield at the same time
window mode with double zoom (CTRL+D to toggle) (MS Windows only)
resolution and refresh rate selection for fullscreen mode
colourful newspaper after a certain date
colour coded vehicle profits
game speed increase (through fast forward button or by pressing the TAB key)
snappy and sticky windows that always stay on top and neatly align themselves to other windows
more hotkeys for even less mouse-clicking
OpenTTD requires the original version of Transport Tycoon Deluxe data files in order to function.