The latest version 0.14 of the game is now declared stable. It took us 8 months since the release on Steam Early Access. We took our time to deliver many new features, usability improvements, and also to fix all of the community reported bugs we could (and it has been a lot).
Below are some of the major additions to the game since we launched on Steam Early Access:

Reworked Multiplayer
The multiplayer module has been completely redone, with focus on speed, scalability, and stability, along with improvements to the general user experience. The servers can now support games with hundreds of concurrent players, such as this event hosted by a player: Factorio Mass Multiplayer Session 2. Big events like these gave us a lot of feedback for user improvements, in many areas of the multiplayer game. Browsing and joining public or LAN games is now done directly from the in game dialog. We worked a lot to make the process of creating, joining and playing the game, even with a large amount of other players, as smooth as possible. As a small bonus there is now the Production challenge Multiplayer scenario shipped directly with the game.

Train System improvements
The train system has seen a large number of changes, both to the gameplay and user experience aspects. The model and sprite for the locomotive and cargo wagon have been completely redesigned, there is a new locomotive gui and train stop gui, a rebalanced max speed and acceleration system, and a new train overview screen. Laying new rails and junctions has been semi-automized (This is a game about automation, remember?). All these changes combined make train systems of various sizes much easier to build and manage.

Steam achievements
The game currently has 38 achievements you can earn, including speedrun challenges, production milestones, and limitation based accolades. If you are a completionist player, these will really give you something to strive towards.

Technology tree
The new techonology gui allows easy and intuitive viewing, choosing, and browsing of technologies and their prerequisites. This mod compatible system allows even the most convoluted research paths to be displayed in a simple format, making the choice of which path to take much easier.

Circuit network extension
The number of possiblilities with the circuit network has been exponentially increased with the addition of more circuit connectable entities. The most powerful features of this extension come from the ability to connect transport belts and inserters to the circuit network. You can also connect train stops, accumulators, power switches, gates, rail signals with more to come in the future.

Much more
Apart from all this there are other things waiting for you in the update: flamethrower turrets and fire spreading, more realistic map generation, in-game accessible mod portal, the blueprint book to organize your blueprints, power switches, and many others to discover...

We still have more updates planned for the future, you can checkout the plan on our forum.