Based on Transport Tycoon Deluxe, the free and open source OpenTTD has release version 12.0 as the multiplayer update.
The big thing with this new release is improved multiplayer and it's much less of a hassle to setup. You no longer need to mess with port-forwarding. The game now does pretty much everything for you. All you need to do now is setup a server, share a code and your friends can join in - nice.
What they've done to manage this is pretty clever. They took inspiration from Steam and other services on how they handle connecting people together. The game will first attempt a direct connection, if that doesn't work it will try through STUN networking and if that fails it will use their own freshly made relay service to go through their servers. It means nearly everyone will be able to connect together but it will increase their costs which they hope will see more donations.
Other features in v12.0 include:
- Display icon/text whether vehicle is lost.
- Moving camera on title screen background.
- Hide block signals in GUI by default (you can toggle this in the settings).
- Raise the maximum NewGRF limit to 255.