What are your favorite monitoring tools you prefer to use to monitor your network ?

Solarwinds Network Monitoring

All SNMP tools has almost same functionalities but differs in the presenation of the data 

Solarwinds Network Performance Monitor

Wireshark ;)

Nagios

There are broader Net monitoring tolls  such as Spectrum and Concord Ehealth , Ehelth is very useful for generating Bandwidth utilization reports

“what’s up”  monitoring software. 

I use Spiceworks and Wireshark. Spiceworks is a wonderful tool if you configure it properly. The easiset way to get your asset list/configuration/validity/warrenty etc.

Spiceworks is Excellent.

Linux Syslog if your linux machine is working as router and you are logging your iptables

Network monitor is also good.

Wireshark and Cacti are the best monitoring tools to monitor. Because both of them has extra ordinary options and tracker's compared to the others. 

Cacti, it's free, simple, easy to use and give many options of monitoring.