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.
I've spent a lot of money (mine and clients') on different tools, and I've settled on Keynote. It covers 95% of my needs and I don't have to manage yet another tool.
Being able to sit in a meeting wireframing on Keynote is wonderful. Factor in another $15 for the iPad app and I'm cooking with gas.
Meetings get super-focused quickly when the participants can immediately see output.
I usually upload it to Google Docs post-meeting for the remote participants.
Protip: Save your mockup files as themes so that you can get to them quickly and easily.
side note: good looking mockups are dangerous because many managers and stakeholders don't know the difference between mockup and real application. They think you're done.
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.
Photoshop for me as well, because I can then easily transition from the wireframe to the high-fidelity mockup. I much prefer it when the evolution is a continuous process, rather than being interrupted by switching software.
I used to feel that way, but when I began using separate framing software I was pleasantly surprised by the results. Having to start the design phase from scratch in a new program freed me from slavish adherence to the frames and allowed designs to progress further. Horses for courses though - I'm sure everyone has their own take.
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.
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.
The Flairbuilder developer is quite active and responsive about updates. I switched here from Balsamic because Flairbuilder allows for somewhat dynamic & functional mockups.
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.
Aside: Creature House Expression was a wonderful - yet industrial-strength - 'painterly' vector drawing package. Superb UI (which many users complained wasn't 'Adobe' enough), and a fun, quirky website. The combination of science and art made it one of the highlights of successful software development. You can guess what happened when Microsoft acquired the company... ;-)
Anyway, MS were kind enough to make the Windows and Mac Classic/OSX versions available for free - follow the wikipedia links.
Just remember, this is software that was a rival to Adobe Illustrator, and contained technology that some thought would threaten Flash. Steep learning curve (but excellent PDF manual included), and capable of far more than UI mockups. The old Topica mailing list is a mine of useful information, and Yahoo has a current (but less trafficked) list.
(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.