We use Balsamiq Mockups for this purpose, it works very well for us and there is a free version.
The mockups look very much like hand-drawn wireframes, which is useful for avoiding the impression of a nearly-complete application.
One advantage of Balsamiq over some other wireframe tools we have tried is that it lets you simulate simple application logic, such as click on this link, and it will load the next web page. This allows simple "demos" of the mockup.
Also (not sure if this just the paid version which we have) it allows you to export your wireframes to PDF, with the clickable links included. this is a nice portable way of showing your mockup to clients, other developers, managers, etc without needing any additional software or anything.