Even with a modest number of devices on your network, it's well worth considering spending your money on WhatsUp. Quick to get started, it's very customisable, and does much of what the full-blown monitoring applications do but at a fraction of the cost.
WhatsUp is a relatively inexpensive Windows-based system monitoring tool suited mainly for small to medium organisations.
It comes in two flavours -"Professional" and "Professional Premium Edition" - the difference being that the Premium Edition does everything its sibling does plus specific monitoring of SQL Server and Exchange installations and some extra support for WMI (Windows Management Instrumentation - Microsoft's management API).
Installing the product is easy even though it ships with a fully-blown installation of the MSDE database engine (basically a slightly crippled version of SQL Server). You just run the installer, answer the few questions it asks, and leave it to chug away. This takes a few minutes.
In order to monitor systems, you need to tell it what to monitor and how. You can do this in a number of ways, from a simple subnet scan (you give it an address range and it goes to find machines in that range) to an "intelligent" SNMP scan which interrogates an SNMP-enabled device such as a router and figures out from its content where to go and look for other devices.
You can, of course, also nominate individual devices if you know their addresses. The devices WhatsUp knows about are collected into a couple of default groups ("All devices" and "All routers") but you can define your own groups and drop things in them yourself.
Different devices have different characteristics, of course, and so for each thing on the network you can tell WhatsUp to monitor different stuff. So you'd maybe enable ping, HTTP and HTTPS for your web server, and ping, SMTP and POP3 for your email server.
As well as monitoring service availability, you can of course also tell it to interrogate the devices for CPU utilisation, network interface usage, disk availability and the like. Rather cleverly, you can also define your own parameters to monitor; just tell it the device and give it the right authentication details to allow it to connect, and it'll figure out the list of stuff that's available for monitoring and let you pick the one(s) you want.
The GUI is the usual two-pane setup, with a list of devices on the left and the detail pane on the right. The latter has three views, which you switch between by clicking little tabs: the Device View is a list of devices monitored; the Map View shows you icons for your various devices and lets you draw a pretty picture of how they all interlink; and the Report View is where you can look at the detail of what your world is doing.
There are shedloads of standard reports, including the obvious stuff like "top ten" lists and single-page summary information for specific devices.