The Windows 8.1 Start Screen is populated with tiles to apps and a link to sign out, lock Windows, or switch users. You can, but there's not much you can do to add features or functionality.
Start Screen Unlimited is a Windows 8 program that adds a few useful features to the Windows 8 Start Screen, like date and time, a link to the Windows control panel, and more power options. It also adds a Google search bar directly to the Start Screen.
After you install Start Screen Unlimited, you'll see a toolbar in the upper right-hand side of the Windows 8 Start Screen. If you press on the infinity logo, you can move the toolbar around the screen. The power options include hibernate, sleep, reboot, and power off. The configuration options display a link to the control panel and allow you to close the toolbar. The "i" button allows you send feedback, buy the deluxe edition, and check for updates.
The Google search bar can be used to search directly from the Start Screen, but it requires a physical keyboard. The official Google search app for Windows 8 brings up the Touch Keyboard automatically, making it a better choice for touch-screen users.
The lite edition of Start Screen Unlimited is free. Buying the deluxe edition allows for commercial use and adds additional menu customization options.