Open TTD is an open source clone of the original transport tycoon deluxe.
ive been playing it like mad recently it has loads of great features and its only at version 0.3.6 just now.
very soon a new graphics engine will be introduced, and judging by some of the vehicle designs on the development forum, it will look amazing.
the source code is also provided on the website including guides on how to compile it and change things. anyone can submit changes to be included into the project.
http://www.openttd.com/
it has loads of new features:
Features
OpenTTD is modeled after the original Transport Tycoon game by
Chris Sawyer and enhances the game experience dramatically. Many features were inspired by
TTDPatch while others are original.
Significant enhancements
=========
- autorail build tool
- canals/shiplifts
- larger stations
- non-uniform stations
- ability to join stations (of same width or length)
- checkpoints
- mammoth trains
- multiheaded trains
- Build on slopes and coasts
- goto depot order
- longer bridges
- higher bridges
- two new bridges
- presignals (CTRL-click on existing signals)
- semaphores (CTRL-D to place them)
- order sharing and copying ("goto" on other vehicle to copy, ctrl+"goto" to share)
- TTDpatch compatible Nonstop-Handling
- crossing tunnels
- build many trees on one tile
- bribe the town authority
Better useability:
================
- convert rail tool (to rail, monorail, maglev)
- build stations with drag&drop
- build while paused
- Mouse wheel can be used to scroll in menus and
to zoom in/out on win32
- autoscroll (to left/right)
- bigger demolish tool
- plant trees on area
- sell whole train by dragging to dynamite trashcan
- cost estimation with Shift
- road and rail removing by dragging
New options:
============
- "no train service" option
- "no inflation" option
- "full load any" option, as in ttdpatch
- Feature: "select goods" option
- "multiple similar industries in close proximity" option
- "multiple industries per town" option
- news item for "train is unprofitable" (change in openttd.cfg)
- news item for "train is lost" (change in openttd.cfg)
- starting_date, takes a value on the form yyyy, yyyymm or yyyymmdd.
- select what vehicle types the ai will build
- ability to chose snowline height
- change default servicing interval for vehicles
- change max # of vehicles per player
- new pathfinding algorithm for trains (enable with new_pathfinding)
Graphical/interface features:
===============
- screenshots can be BMP, PNG or PCX (select in game options menu)
- More currencies (including Euro introduction in 2002)
- More townname languages
- window mode
with double size mode (ctrl-d to toggle) (only Windows)
- resolution selection for fullscreen mode
- refresh rate configuration setting
- town directory sorting
- patch options configuration window
- "debtmax" faster loan management with ctrl key
- colorful newspaper after a certain date
- game speed increase (through fast forward button or by pressing Tab)
- color coded vehicle profits
- plant random tree type
- Hotkeys
The Disclamer:
This game requires an original copy of transport tycoon deluxe to run. this game is commonly considered abandonware, but is still copyrighted by microprose.
the original game can still be bought though (currently for £1.47 + delivery - very cheap!)
http://www.amazon.co.uk/exec/obidos/...324917-6928618 
