System Monitor II
System Monitor II is a very useful gadget that provides a quick overview about your system, providing a bunch of useful details directly on the desktop.
Since it's a gadget, System Monitor II can only be accessed from the desktop, but even so, it provides vital information in a much more appealing way than many other standalone utilities.
System Monitor II displays memory usage, but also the used, free and total amount of memory, with detailed statistics shown right in the main window. Additionally, it displays CPU model and speed, overall and per core usage, alongside a professional-looking graph to help you keep an eye on the way the system works.
What's more impressive is the configuration screen that includes tons of options for a Windows gadget. Besides the fact that you can customize interface options such as colors, background and graph details, System Monitor II allows you to specify the CPU you wish to monitor, the refresh rate, core temperatures and memory options.
Graphs can be configured as well, with multiple built-in options, so you may have to spend a while setting up both the appearance, as well as the other features of the widget.
Of course, because it's a widget, the footprint on hardware resources is minimal; it works exclusively on Windows Vista and 7, since they're the only ones boasting a sidebar.
Overall, System Monitor II is undoubtedly a must have gadget and thanks to its rich features, it's much more useful than many stand-alone system information tools.