I can't imagine monitoring 1000's of devices with WhatsUp Professional or Adventnet OpManager or any other "retail" box-off-the-shelf product. And the idea that there aren't commercial support options available for open source products like Nagios / OpenNMS / Zenoss is simply ridiculous. Show me any available commercial or open source solution that can monitor 1000's of devices on a single server effectively - it doesn't exist ! For large numbers of devices you need distributed monitoring and unless you have more cash than Bill G, forget about OpenView or Unicenter etc. Find a reputable company that will provide you with an open source based solution. In the long run you will get a better return on investment from open source than closed, proprietary software.