Balsamiq desktop, they also have an atlassian plugin for using within your JIRA job ticketing or a web only version
Balsamiq is good because there is not too much functionality there to get carried away with, and the hand drawn style prevents stakeholders believing it is almost finished and might look like that when launched.
Oh, and there are sites out there as well which will scrape an existing site and turn it into a balsamiq wireframe. So if you say liked the way the new twitter looks, you can scrape the layout, alter it and then provide that back out to your dev/ui teams
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