Xenonauts is now available on Steam Early Access, and with that has formally entered Beta. If you have pre-ordered Xenonauts at any point prior to today, you are entitled to a Steam key (and you'll also get one if you buy the game on Desura in the future too).
To claim and activate your Steam key:
1) Go to Secure.desura.com
2) Log into your Desura account
3) Click the "Request Key" button next to Xenonauts
4) Load Steam and go to Games > Activate A Product On Steam in the topbar
5) Copy the key into Steam
The above steps are also viewable in picture form here: Xenonauts.com
However, if you would rather stay with Desura, that's fine. We will continue to release new builds on Desura as we make them available on Steam, so it should just be down to personal preference for users.
Build V18 Stable should be out on Desura within the next 24 hours, and is our first Beta build. The game can be played all the way up to (but not including) the final mission and the vast majority of the game mechanics are now in place. What comes now is balancing and polishing it, improving the graphics and making all of the little changes we'll need to make to say the game is complete. However, it should still be playable and hopefully enjoyable too!
Mac and Linux versions of V18 Stable will be arriving shortly both here and on Steam too!