You can purchase additional USB or Serial cradles (available in graphite or ice) for $29.95. There is also a USB cable for mobile users for $19.95.
Don't like the plastic stylus that comes with your Visor? Buy the Visor Stylus 5-pack in a rainbow of Visor colors for $15.95. These styli are metal and should not feel light and flimsy. There will also be a pen stylus available in November.
There are three types of cases that you can purchase: Premier cases, which start at $39.95, matching cases to match the color of your Visor, and Value cases for the more frugal buyers. You can also make a custom case with your initials.
There are also other goodies (T-shirts, hats, etc.) at Handspring's site as well. The site is a little slow right now, but should improve once the initial rush has subsided.