I'm looking for something that applies the collaboration focus (one location/URL, always up-to-date, multi-person online read/write access anywhere) of google docs / google spreadsheets to wireframing. Bonus points if, like Google Docs, it needs only a browser yet also works offline. More bonus points if it supports automatic revisions. Even more bonus points if you can hand out login-less 'invitation' URLs like Flickr does, instead of forcing people into signing up for accounts or using their home accounts.

To start off, there's one called iPlotz, but it didn't enchant me -- ironically, mostly because of its akward UI, which can't hold a candle to omnigraffle (don't let that prevent you from giving it a try though).

And no, paper prototyping, wonderful as it is, does not qualify: it does not combine being instantly globally shareable & editable very well :-)