Twine is a tool for creating non-linear stories that can be played in HTML. Here are some examples. It's available under GPL and runs on Linux, Mac OS X and Windows.
There's also a tutorial available by a game designer I follow.
Beginning this Thursday, there will be a one-week-long Virtual Game Jam in a community of gamers that are feminist, queer, disabled, people of color, transgender, poor, gay, lesbian, and others who belong to marginalized groups, as well as allies. They have an IRC channel.
I will give this tool a try at the next Berlin 8-hour Jam.