GeekTool is an application for OS X 10.6 or later. It lets you display various kinds of information on your desktop via 3 default plug-ins:File plugin to monitor OS X activity with /var/log/system.log, or any file that you want to follow.Shell mode to launch custom scripts or commands like "df" to check space left on filesystems or "uptime" to monitor load on your machine. Image mode helps you monitor bandwith usage, CPU load, memory availability on your server with tools like MRTG or RRD.
Monitor real-time power usage on your Mac computer.
Monitor and log all the temperatures and fan speeds inside your Mac.
Set wallpapers that include the date and time.
CoolBook lets you monitor CPU frequency, voltage, and temperature.
Background image changer application.