Keynote for Mac. I love the fact it's so easy to use. And sometimes the mockups look so good you'll want to "lick them", to quote Steve Jobs. :))
Balsamiq
Photoshop. I've become so accustomed over the years to the tools/interface, that it's actually faster than pen/paper. Setting it up with common UI elements to drop in quickly makes mockups a breeze.
The most efficient thing for me has been Illustrator w/ wireframe symbols. It's not so low fidelity that I can't envision the end products, additionally I enjoy the control vs. mockup tools like Balsamiq.
There some good wireframing resources for Illustrator and other programs in this Smashing Magazine post: http://www.smashingmagazine.com/2010/02/05/50-free-ui-and-we...
Wireframe Sketcher in Eclipse http://www.wireframesketcher.com
Google docs, believe it or not. I found a nice wireframing set at the following address: http://mortenjust.com/2010/04/19/a-wireframe-kit-for-google-...
There're also SketchyPad and iMockups. I've created a list of the tools presented here, you can see it on http://www.romku.com/list/343849/ui-mockup-tools.
Thanks again for all the feedback. I've been trying various tools and i've honestly been loving flairbuilder. Quick, simple to use, and it's highly interactive that's just built in. Haven't tried balsmiq yet but will give it a shot.
as a product manager, I use 2 tools for wireframes and flow design
Flairbuilder and OmniGraffle http://flairbuilder.com/ http://http://www.omnigroup.com/products/omnigraffle/
The Flairbuilder developer is quite active and responsive about updates. I switched here from Balsamic because Flairbuilder allows for somewhat dynamic & functional mockups.
A whiteboard, or Penultimate on the iPad if there's no whiteboard around.
I love Pencil for Firefox. It's seriously underrated: https://addons.mozilla.org/en-US/firefox/addon/pencil/
Omnigraffle, Photoshop or paper. Although, seeing the templates created by Amir Khella (keynotopia.com) has made me want to try to use Fireworks a bit more for this.
Mockingbird - https://gomockingbird.com/
Maybe it's just because I love web apps done with Cappuccino.
Trying the templates in Keynotopia, they are good, but I think I need to work longer on paper before spending time in PowerPoint.
Sharpies and copier paper. Seriously: nothing else has been as efficient for me. (Although it depends on your Sharpie skillz.)
Mockingbird - https://gomockingbird.com/
I have tried Pencil, Balsamiq, MockingBird and Google Docs. Google Drawings works best for me.
Fireworks
A piece of paper.
Paper first, a Tcl/Tk mockup second.
Pen and paper then Inkscape/Gimp.
in order of preference: 1. pen + paper 2. html + css 3. omnigraffle 4. photoshop
Paper 1st and then Keynote.
Balsamiq
Paper, Illustrator, HTML.
Expression Sketchboard
Photoshop
html/css/js
I've gone through a bunch - Illustrator to Balsamiq to Mockflow and now HotGloo. I like Mockflow and HotGloo (both online fee-based apps) because they offer:
(1) Master Pages (or at least Master Objects) to keep things DRY (2) a way for clients to view, navigate, and comment directly on the frames (3) a good balance: low-fi enough to avoid seeming like a design pass, but hi-fi enough that each object is distinct and recognizable (4) a good selection of resizable, customizable widgets (5) an environment separate from the design file. Unlike many, I actually prefer to start from scratch with the design - it frees me from conforming too tightly to the frames, and leaves open the possibility for further iteration. Also, it's handy when passing frames to a visual designer - they can use whatever design tool they like.