Do you use Safari 4 and start.io? Here’s how you can change your Home button into a startpage button:

• Download and unzip the required assets.
• Quit Safari.
• Go to your Applications folder, right click Safari and choose “Show Package Contents.”
• Place the file ToolbarHomepageTemplate.pdf in ~/Contents/Resources/. Be sure to backup the original file (the zip includes a backup as well).

Safari should now have a snazzy .io button. This is completely not supported; customize at your own risk.