Balsamiq or mockflow.com (Basic version is free but its kinda useless)

Also, go here for a good list of wireframing tools: http://www.smashingmagazine.com/2009/09/01/35-excellent-wire...

Scroll half way down and look for "2. Wireframing and Mockup Tools"

See here for a list of wireframing/prototyping tool: http://c2.com/cgi/wiki?GuiPrototypingTools

plug: Try App Sketcher (http://www.appsketcher.com) if you need to show interactions in your prototype. App Sketcher creates prototypes in HTML/CSS/JS (no coding needed) that can run in any browser.

LucidChart (http://www.lucidchart.com)

I love being able to use it anywhere, and they just updated it to work on the iPad which is nice. It's got a few issues to iron out, nothing that's kept me from getting things done, and I like supporting these guys.

PencilProject (http://pencil.evolus.vn)

It provides a decent set of features and has a browser version, as well as a desktop version.

+1 for LucidChart (http://www.lucidchart.com). I'm a big fan of the collaboration with clients.

http://www.flairbuilder.com is the best.

Axure (http://www.axure.com/)

you can use papasketh, it is a powerpoint like tool, can make interactive wireframes, very cool. http://www.papablue.com

- Pen + Paper - Adobe Fireworks - Adobe Illustrator

Google Docs Drawing tool

Very simple and quick to throw up some ideas

pen + paper

+1

omnigraffle

I also use Omnigraffle, hands down best app for detailed wireframing. Sometimes it can be too much. In those cases I'll just use Mockingbird