[hammerdr]

I use Mockingbird for wireframing. http://gomockingbird.com

It doesn't allow collaboration but it is entirely focused on wire framing (unlike GDrawings, OmniGraffle and Visio). It provides a great experience.

For everything else, I use OmniGraffle.

[saikat]

And we should be getting collaboration soon - I'm going to start working on it tomorrow.

[brown9-2]

mockingbird is great, I love the "look" of it's graphics