Star Citizen is offering everyone a free week, just like it did with PAX East 2015 goers last week. Be warned, the demo client is 20 GB.
To get started, one needs a Star Citizen account (or to create one). Then, enter the code SXSWFREEFLY2K15 at this site and get to downloading. (Correction: An earlier version of this post gave an incorrect code. It's also a full week, not a weekend. We regret the errors.)
The demo allows players to fly an Anvil Hornet F7C trainer ship in the Arena Commander space combat module, and walk around the Hangar Module in first person mode.
Twenty gigabytes is a hefty download for a demo; it's also one-fifth of what Cloud Imperium expects will be a 100GB main client when the completed game finally launches sometime in 2016.