GKrellM is a stack-able widget like system monitor that allows theming.
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Pro Many available plugins
The uses of the monitor can be expanded with available plugins ranging from weather information to TIVO information.
Pro Set up alerts
Through the settings one can set alarms and warning for each monitor, which allows one to always stay informed if anything is getting above what their set limits are.
Pro Customize what to monitor
Users can easily set GKrellM to monitor what they like (depending on what monitors are supported by their PC) such as CPU usage (which can be set to monitor each core individually), as well as temperatures and fan speeds.
Con Default theme is not that great looking
The theme that comes by default for GKrellM is not really appealing to the eye and sadly many of the available themes are not that much better with fuzzy text and bad color choices.