(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.
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.
Usually the mockups are better than the final product, because in Keynote 1. you can align everything so damn good 2. the font rendering looks awesome
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...
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.
I do believe Firefox's frenetic update schedule just bit me in the ass.
EDIT There's a standalone version: http://pencil.evolus.vn/en-US/Downloads/Application.aspx
EDIT2 This add-on ("temporary branch") works with Firefox 4-6: http://code.google.com/p/pencil-ff4/
Maybe it's just because I love web apps done with Cappuccino.
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.
