How to set up a web-based lightweight system monitor on Linux

Sometimes we, as a normal user or a system admin, need to know how well our system is running. Many questions related to system status can be answered by checking log files generated by active services. However, inspecting every bit of log files is not easy even for seasoned system admins. That is why they rely on monitoring software which is capable of gathering information from different sources, and reporting analysis result in easy to understand formats, such as graphs, visualization, statistics, etc.