What version is the Kerbal Space Program demo?

According to their blog, the latest version of the demo is based off of 0.18.2. This means you have docking, which was added in .18. There are some restrictions from the full version, though:

• Part loading limited to the 25 stock ones.
• No Plugin loading.
• SPH and Runway Facilities are inaccessible.
• Only one save profile allowed.
• Cannot launch new flights if 3 or more are already in progress.
• Kerbin, the Mun and the Sun are the only celestial bodies in the solar system.

You should be able to see the version in the bottom right corner of the main screen, once the game has loaded. Currently, you're at least two versions behind.