I found an interesting web-hosted application called ProtoShare that allows you to create clickable and working wireframes very easily. It has all the HTML elements you would need to design a web application, like headers, text areas, form interaction and even drop-down XHTML menus. Once you have a design, you can share it with others. It also creates a basic, linkable template that allows you to preview usage flow and overall experience.
This screencast demonstrates how ProtoShare can be used to create wireframes.