This great tool that I blogged about a while back has some new features in the pipleline. The downside is that you'll have to pay for them.
My previous post about Mockingbird, with a demo I knocked up
The new features will enable a community of users to work on a prototype together which is fantastic. Communication and evolution of design is what prototyping is all about at the end of the day.
The entry level package seems very reasonable at $9 per month too. Any month you're not using it, you won't get billed - which is perfect for most of us who prototype sporadically as part of projects.
The free service will remain, with up to 10 pages allowed in 1 project. From their blog it sounds like they're listening to their user group as they set this, which is nice.
The currently available beta version runs til 15 August after which the new features come in.
Sign up now though - currently they're saying that beta users will get a more generous free account than those who register later.
Mockingbird online prototyping tool