Monitoring in a Data Center

I was thinking what we need to consider for a thorough monitoring strategy in an IT environment like a Data Center. There are various areas that should be monitored to meet High Availability. Monitoring can be categorized as follow:

• Physical Access
• Environmental Parameters
• Hardwares
• Bandwidth Usage
• Server Connectivity
• Server Resources Usage
• Service Availability
• Traffic Analyzing
• Log Analyzing

For each monitoring category there are many tools available both commercial and free (Mainly Open Source) that can be obtained and implemented. Some tools might cover more than one of the above categories and some just limited to a single category.
Having all of these monitoring services and procedures in place might be too expensive for some organizations and based on priorities and needs one or some of the above might be picked up.
I will post more about monitoring later.