I'm a consultant that has around 100 clients and I would love to monitor all my clients networks, preferably from the cloud so I can tell if their Internet is down. I would like for it to offer a lot of the same monitoring features that send alerts (similar to Microsoft's SBS product). What's out there? Who is the defacto standard when it comes to this? I just need some options so I can know what is going on at my client's sites!
There isn't one standard, at all. You need to define what you're monitoring for your clients and how you'll do it. If you're making sure their websites are up, a hosted solution (is that what you mean by "cloud") is probably best. The hosted monitor will hit the websites from one or multiple locations and measure the response times.
If you're doing infrastructure management or "IT Managed Services" for them, then you need to monitor their internal systems, and you probably won't be doing this from "the cloud." (Note last paragraph however.) You need VPN or other private connections to their networks and you will want to run a monitoring and alerting product from your own network. Could be free stuff like Nagios and Big Brother, or could be top-dollar software like HPOV or Cisco Works.
If their internet connection goes down, you'll get red lights all over the place because it will look like every device dropped, so that covers that. The downside is that if your internet connection goes down, you're no longer monitoring anything.
I do understand that there is some software now for Managed Services companies that involve less network requirements and just operate over agents, possibly without a VPN required. Kaseya is a name I've heard, but have no hands-on experience with.
I would offer two suggestions for this:
1) Nagios: You can monitor as much or as little as you want for each client.
2) Spiceworks: They have made large strides lately running remote collectors and keeping tabs of network links / sites. This option works without a VPN and runs over SSL.
For either option you can run it on EC2 or which ever cloud provider you wanted.
Have you looked at Cloudkick?
Do you need to monitor server internals such as CPU or disk space? Then try cloudkick. I did not use it, but they run ads everywhere ;)
If you only need remote website monitoring, then I recommend AlertFox and Pingdom. These are powerful low-cost solutions.
Chartbeat is a good option if you need analytics as well.
For monitoring system internals (disk space, event logs, etc) PA Server Monitor can monitor a remote client site without needing holes in their firewalls or agents installed on every machine. It uses a satellite monitoring service (essentially an agent installed on a single computer that monitors the rest of the site), and can report back to your central monitoring system at your office.