Gkrellm is a system monitor that is written in gtk.
It has quite a few useful features, , monitor cpu activity, memory and disk usage statistics, you can also view how your ethernet ports are being used. One feature I also found was that it can also monitor your e-mail inbox (see the very bottom) and it will let you know when a new e-mail comes in, click on the button and your e-mail client will pop up. The memory usage stats include both swap and normal RAM usage
You can download from the gkrellm website. There are various options such as plug-ins for more features and themes to change how it looks.