Recs.
Updated
Xfce is a low resource user friendly desktop environment created for UNIX-like operating systems.
SpecsUpdate
Pros
Pro Low system resource consumption
Not just helpful for older computers where few system resources are available, but also simply for those who want to get the most out of their systems.
Pro Classic and familiar
Xfce works very much like the classic Gnome & Windows desktops, taskbars (panels) and desktop icons, letting you get your work done without being frustrated.
Xfce embodies the traditional UNIX philosophy of modularity and re-usability. It consists of a number of components that provide the full functionality one can expect of a modern desktop environment. They are packaged separately and you can pick among the available packages to create the optimal personal working environment.
Pro Adheres to standards
A priority of Xfce is adherence to standards, specifically those defined at freedesktop.org allowing for interoperability and shared technology for X Window System desktops. This interoperability is particularly significant for users looking to, e.g., run alternative window managers.
Pro A true UNIX Desktop Environment
Xfce adheres to the UNIX philosophy, which means it strives for being modular, minimal and expandable. This makes it very much customizable. You can make it as minimal as you want and as heavyweight as you want depending on the features and modules/plugins you use.
Pro Window manager (XFWM) is a compositing WM by default
By having a compositing WM as the default WM makes way for a lot of visual tweaks and tricks that can and do make Xfce look great. You can adjust the transparency, shadows, borders, etc. and many other advanced tweaks are also available.
Pro Sexy layout - while staying slim and unix-ish.
When leaving KDE and GNOME, I was afraid to loose the "wow-effect" it had for first-time Linux viewers/users when going for a more resource-friendly WM.
XFCE with compositing effects never failed to impress - and I don't feel like I'm missing out on anything. Happy :)