I’ve started using Balsamiq to Mockup Salesforce pages which contain Visualforce components embedded within the Salesforce Standard UI Page Layout.
Balsamiq proves to be efficient to conduct such activities in a time effective manner. You can see on this example that I’m comparing the standard Salesforce UI for Contacts and the same Contact page with a Visualforce component embedded inside.
Balsamiq offer a client version and an online, browser based version of the service. Give it a try if this makes sense. I’d be glad to hear about it.
You will find here 2 BMML files, which is the Balsamiq file format. Simply load these files into a new Balsamiq project.