When I was making the mockup for the first time, my colleague told me to use Excel. The mockup is too simple, and cost a lot of time. This way is not better than directly drawing on the paper.
More than one year ago, a colleague introduced Balsamiq Mockup to us, everyone was very happy to use it. It cost less time and looks very similar with the real html prototype, and very lovely.
It can also do iPhone mockup, it's very exciting for the iPhone development guys.
Have a look what Balsamiq Mockup could do:
It can aslo works on mac, linux OS.
Introduce form other's blog: http://www.hanjunxing.com/balsamiq-mockups-one-for-all
more informations here:
Website:http://www.balsamiq.com/
Online Demo:http://www.balsamiq.com/products/mockups
Trail vertion:http://www.balsamiq.com/demos/mockups/Mockups.html