Conky is a lightweight system monitor for the X display server that is fully customizable and able to be displayed on ones desktop similar to a widget.
Ranked in these QuestionsQuestion Ranking
Pro Infinitely themeable
Conky is editable to the point that there is almost no limit to the designs one can use to display the type of information one wants. Not only are there thousands of themes available on sites like deviantart and gnome-look, but a user can create any type of look they like with a little bit of know-how and imagination.
Pro Many different sensor stats
Conky has a bunch of sensor stats that the user can choose to display such as CPU usage, memory, uptime, etc. Depending on what the user wants to display, most likely there is a way to display the needed info when using Conky.
Pro Runs on top of X
Con Requires X
Cannot easily be used on a headless server or remotely (most people and servers support remoting via SSH).
Con Setup can be difficult
Being text based, the config file for Conky can be intimidating for those unfamiliar. Though there are configuration managers out there such as Conky Manager which can make the setup a bit easier.