Dashcode is a new application for developing Dashboard widgets coming in Mac OS X 10.5 (Leopard). This tools is already available as developer preview on Tiger.
I have installed it and it is really great, easy to use, helping you to create good looking widgets. As usual, like all the Apple development tools it is really intuitive. More than a long blog entry on this I am just pointing you to the DashCode page on the Apple site: