There's a nice round-up of wireframing and mockup webapps over at SpeckyBoy. I love OmniGraffle, but most of the time I need it on my Windows machine at work. I tried most of the apps in the list and only one was good enough for my needs. Specifically, NO FLASH REQUIRED.
Mockingbird is impressive just for how attractive it is without using any flash. I can imagine a number of use cases for an app like this. It's a smidge pricey though. The cheapest paid plan is $9/mo but the free plan is very useful.