Agent-based server monitoring tool that can phone home over the Internet

My company has been deploying so many (Windows) servers that it's difficult for admins to manually keep track of disk space, service status, etc. on every single box.  They're also located on many different networks, so we have to VPN to each network to administer the servers at that site.  

I'm wondering if there's an agent-based utility we can install on each server that will monitor the basics (disk space, services running, etc.) and email us an alert if certain events occur (service stops) or thresholds are met (disk is low).  Ideally, each agent's status could be reviewed on a web-based portal, like LogMeIn does, without needing a VPN connection or anything else besides Internet access.  It would be even better if the agent didn't use email to communicate with us directly, but rather contacted a web service that emails us, so we don't have to rely on one of our mail servers staying up for alerts to function.

Management doesn't want to pay for a tool like Kaseya--if that's even a good fit--so something inexpensive or free would be ideal.  We're also prohibited from "rolling our own" tool with scripts, etc.; this would need to be a commercial/third party tool.  I'm not sure this is a realistic proposition given our needs.