We used this to mock up our example adventure game in the workshop. It has a great user interface, and can display graphics. Playing sounds can be problematic.
Free download from twinery.org (though do consider giving them some dollars for development.)
Similar to Twine, though interface is a little more cumbersome. Displays images and plays sounds very easily.
Available from textadventures.co.uk/squiffy (you may need to make a profile at textadventures.co.uk first. Example games can be played through that site too.)