There are a lot of wireframing tools (online/offline) offered in the interwebs. One that stood out for me is the Pencil Project from Evolus. Aside from being free, Pencil is easy to use, has inter-page linkings, has sketchy stencils, allows clipart browsing and more.
The standalone build (offline)/version of this app is available for GNU/Linux and Windows. This is also offered as a Firefox addon making it also available for Mac OS X users. However, the Pencil Project 1.2 addon only works on Firefox 3.5+. If you’re a Firefox 4 user, you may download this tweaked version of the addon. Check out this blogsite if you want to see how the tweak is done.