Balsamiq Mockups (http://www.balsamiq.com) is an awesome way to make mockups. They also recently introduced a way so one mockup can "link" to another mockup (http://www.balsamiq.com/blog/?p=1181).

You can use this for usability testing on Macs or PCs (it uses Adobe AIR). I bought the license for the desktop version ($79) after downloading it for free. The license lets you load/save multiple mockups.

There's also a web version, although I haven't tried it.