Balsamiq or mockflow.com (Basic version is free but its kinda useless)<p>Also, go here for a good list of wireframing tools: <a href="http://www.smashingmagazine.com/2009/09/01/35-excellent-wireframing-resources/" rel="nofollow">http://www.smashingmagazine.com/2009/09/01/35-excellent-wire...</a><p>Scroll half way down and look for "2. Wireframing and Mockup Tools"
See here for a list of wireframing/prototyping tool: <a href="http://c2.com/cgi/wiki?GuiPrototypingTools" rel="nofollow">http://c2.com/cgi/wiki?GuiPrototypingTools</a><p>plug: Try App Sketcher (<a href="http://www.appsketcher.com" rel="nofollow">http://www.appsketcher.com</a>) 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 (<a href="http://www.lucidchart.com" rel="nofollow">http://www.lucidchart.com</a>)<p>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 (<a href="http://pencil.evolus.vn" rel="nofollow">http://pencil.evolus.vn</a>)<p>It provides a decent set of features and has a browser version, as well as a desktop version.
you can use papasketh, it is a powerpoint like tool, can make interactive wireframes, very cool.
<a href="http://www.papablue.com" rel="nofollow">http://www.papablue.com</a>