[userium]

I have used Mockingbird (https://gomockingbird.com/) and Balsamiq (http://www.balsamiq.com/) in the past. Currently I like to use Omnigraffle pro (http://www.omnigroup.com/products/omnigraffle/).

[mrgreenfur]

I use and adore Omnigraffle. It is hands down the best diagraming software I've ever seen.