Browsing DjangoSites, I came across a site called MyDarkSecret - a web-based game comparible to the "How to host a murder" roleplaying party game. And it's free! Aspiring authors can also write their own scripts for others to enjoy.
Mel and I have hosted a "Host a murder" party, and it was great fun although it took a lot of work to organise.
It's early days for this site, and the interface still needs some work, but it's fun. Also, it's a lot less effort (and less costly) than the "real" version.