I've seen it commented before, and I've had a good look myself, but there seems to be a distinct lack of open-source free adventure games. However, it seems to me that reason is obvious - there's not much fun in writing them.
The fun of playing an adventure is wondering what's going to happen next; where the story is going, what you might see. But as the programmer, there are no surprises because you actually have to create it (and test it, again and again). As the programmer, you are the one person who can never appreciate the adventure for what it is, so it would be quite a sacrifice of your time to actually write one.
Still, I did finds this link to free adventures. I've never played them though, so I don't know if they're any good.