Clearspring/AddThis, our creative director Jeff Wong and I use Omnigraffle, as we can frequently move directly from flows, to wireframes, to mocks all in the same tool. Much faster. Also, Omnigraffle handles multi-page PDFs rather nicely. I'm also a fan of their Omnifocus Todo app, but that's a different blog post!
This link from W3Avenue includes a huge collection of resources about Web wireframing:
Guidelines, Tools and Resources For Web Wireframing | W3Avenue
What's your favorite wireframing tool? What's been your experience with web wireframing?