There are some other ones around but I love this as you can work on it with multiple users at the same time.
well, I fell for it lol I went to the website and wasted like 20 mins of my life messing around with it. I get sidetracked so easily lol IT is fun to mess around with though
Hi, I think our soon-to-be-released application might interest some of you ! This will be a paid App but it will definitely make your life easier. Check it out at www.appcooker.com and register to the beta program. We would love to hear from you ! Thanks
Another option for this is LucidChart's iPhone mockup tool. I've used this for a few projects and have been quite happy with it. From looking at the other link, it looks like LucidChart offers a lot more GUI images for the iPhone and also offers real-time collaboration so you can share your mockup with others, or invite other designers/developers to work on it with you.