How to monitor network health?

Thanks for this info everyone. At the moment we are stuck with installing PingPlotter on a desktop at each clients LAN - obviously this solution sucks and isn't scalable to the 100 networks we need to monitor. We have demoed Appneta but at a minimum of $7000 per year for only 3 sites it isn't a good ROI for our limited need. I assume everyone in IT has the need to measure and alert on network health so I feel like I'm missing something. Here's the only choices I see:

1) buy and install Cisco or Juniper equipment and use LM to gather Netflow data

2) use Ping datasource to measure ICMP latency as the only indicator of network health

3) purchase enterprise-grade network health software completely outside of LM

4) invent network health monitor software that runs as a virtual machine appliance and reports Netflow and SLA data (ideal, but non-existent)