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 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
clone, autoreplace and autoupdate vehicles
possibility to build on slopes and coasts
goto depot orders, check orders and view order destinations
order sharing and copying
longer and higher bridges including 2 new ones
reworked airport system with 2 more airports (international and metropolitan)
presignals, semaphores
preliminary support for TTDPatch newgrf features
build many trees on one tile
bribe the town authority
lots and lots of patch- and configuration settings to tune the game to your liking. Also look at the wiki.
save games with zlib compression for smaller sizes which does not interrupt gameplay
big support for internationalization.

http://rapidshare.com/files/88773194/TTD_0.5.2.0.rar