Google has announced some free open source webfonts. They currently provide only a small library of Fonts but even these can add an extra dimension to your Cappuccino app.
Simply add the necessary link tags to your header in index.html and index-debug.html to add the desired font, e.g.
<link rel="stylesheet" type="text/css"
href="http://fonts.googleapis.com/css?family=Lobster">
Now use CPFont fontWithName:size: and you are away!
E.g.
[label setFont:[CPFont fontWithName:'Lobster' size:28]];
Enjoy!