Check out http://www.continuum.net/. I've been using their services for over 5 years, and they've been steadily improving it since they split from Zenith Infotech. No, it's not free, but it's quite cheap per unit and you get a lot of bang for your buck. Remote monitoring and alerts on any service, remote access, at-a-glance dashboard, etc. With 500 clients, I'm guessing you'd rather spend your time monitoring the situation than putting together a custom solution.