<p>I want to draw user interfaces for web and desktop applications.</p>

<p>I need something less print-oriented than omnigraffle. think pixels!</p>

<p>Also, need good building blocks (aka stencils). Form elements, tableviews, etc.</p>

I very much like
Omnigraffle
, with
stencils
for UI design. More UI stencils are
also available
.
You might also check some
other
stackoverflow
questions
on this topic.