What do you think best tools for IT infrastructure monitoring?

It does depends on your infrastructure, but it also depends on your needs.
We use here Spiceworks.
It's free and very useful.
Spiceworks Features: Inventory Your Network Network Inventory IT Asset Management IT Audit Software Warranty Tracking Virtualization Management Monitor Your Network Network Monitoring Power Management SNMP Network Management SQL Server Monitoring Cloud Management Tools UPS Power Management Software Run an IT Help Desk Help Desk Software Hosted Email Support Active Directory Management IT Purchasing Management Request for Quotation (RFQ) Help Desk iPhone App Help Desk Android App Knowledge Base Utilities TFTP Server Change Management Network Mapping Network Troubleshooting MSP Software IT Community It's also possible to monitor Office 365 mailboxes with it.
Email alerts, web dashboard, user-friendly interface, large community with forums...
Out of all this, it can really improve the way you manage your IT infrastructure.

Nagios Is The Industry Standard In IT Infrastructure Monitoring its free also Nagios offers complete monitoring and alerting for servers, switches, applications, and services With Nagios you can: Monitor your entire IT infrastructure Spot problems before they occur Know immediately when problems arise Share availability data with stakeholders Detect security breaches Plan and budget for IT upgrades Reduce downtime and business losses

it largely depends on the infrastructure, here we use solarwinds which is a very powerful tool and has lots of addons (paid and free) it features: network performance monitoring netflow traffic analysis network inventory free IP address manager a bit expensive though

It depends on the infra-structure make.
For example, HP has OpenView, IBM tivoli ..etc.
I know some companies who develop their own customized applications on top of the infra-structure monitoring tool.
So, in one dashboard they could monitor the network equipment as well as the servers.

Questions about best tools for IT infrastructure monitoring is a philosophical question :).

 

If you have huge infrastructure and have budget you can choose HP BSM or ITM, if you do not have budget you can choose just Nagios and good IT specialist. If your infrastructure small you can choose any system. In common functionality all system almost equal. 

Nagios, Solarwind is an all in one pack , where one can add and remove modules ....

I used SpiceWork and it's my best choice.

We use : HP OpenView Nagios Xymon ( for server and some basic services ) Cacti