During the software development process, I've found most clients have a hard time visualizing what you're planning on building them -- even with extremely detailed specifications. In the past, I've used slick product called Balsamiq for building mock-ups of the user interface and business processes. Including these visual aids along with the specs made things a lot clearer for the client as to what they were getting. If something wasn't quite how they wanted it, everything could be quickly modified, long before a line of code was written. The amount of time saved on one project alone could easily justify it's $80 price tag.
As luck would have it, I've run across something even better. Evolus has a similar prototyping tool called Pencil which has the fast majority of the features Balsamiq offers, and then some. Among these are the ability to have more than one mock-up per document (I remember having a whole folder of mock-ups per project when I used Balsamiq). Pencil also offers native interface widgets, in addition to the sketchy style. While Balsamiq offers sketchy only, they do have a lot more widgets.
Pencil is built on the Mozilla Framework, and although I prefer the standalone version, it's also available as a Firefox plug-in. Best part is its price tag: free! Licensed under the GPL (version 2), it's available under Windows and several flavors of Linux. Sounds like a Mac version is also in the works. Overall, I love the software and look forward to using it more.