Spiceworks is a free piece of software I have been using for a while now that can monitor your network. The software runs on a Windows server however you can access the website interface from anywhere.
Spiceworks allows you to create alerts on many things on your network that can email you and let you know if there are problems. You can create alerts if a server or network device is offline. You can also create alerts if one of your printers is getting low on toner.
Spiceworks also has a user portal which allows your users to be able to submit trouble tickets if they are having problems with their computers. The user portal is totally customizable in the back end and you can add anything to it.
Spiceworks has a great reporting tool. You can get reports on all software installed on the computers, disk usage, RAM, CPU and more. The reports are also customizable so you can make you own.
If you click on a specific device in Spiceworks, you can get all of the information on that device including, what antivirus is installed, how much free hard drive space, how much memory and a lot more.
So if you are looking for a network monitoring tool, I would definitely check out Spiceworks.