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There's a tool called Pencil[1] that runs on the browser, but works offline. Haven't explored a lot, but does simple tasks well.

[1] http://pencil.evolus.vn/en-US/Home.aspx

That's awesome! I have searched for an offline tool like that and it looks very good. Thank you!

I swear by FlairBuilder. We've built complex sites using this for prototyping and the learning curve isn't high at all.

What's wrong with Air? Balsamiq works quite well for me, don't notice any problems caused by Air.

It's slow, non-native, and resultantly slightly obtuse and frustrating to use.

Non-native, true. But slow? Balsamiq on my PC is nothing but snappy. You should consider sending a support request in case there is a bug in Balsamiq(or Air?) with your OS/pc configuration.

There are a number of desktop wireframing applications - Omnigraffle, Axure, Visio come to mind.