The more basic scripts for server monitoring don't seem to be that up to snuff. If you're promising monitoring to your clients, you should consider paying for real monitoring.
That being said, if you have a Windows machine lying around, you might find a program called ServersAlive. It's pretty nice.
As for *nix/web based systems, I just decided we'd write our own. It'll be more client-oriented than anything out there now, but a public version isn't likely to be ready for a while. We're using the core of the app in-house while we "flesh" it out.
We have Siteuptime and Alertra working double time on our site, and they both notify us when the site goes down very quickly...Usually Alertra's alert ends up in our box a lil quicker than Siteuptime.com's....
I have Automize from HiTek software (http://www.hiteksoftware.com/mize/automize.htm) running 24/7 on a linux box (also works on windows). It's a highly configurable piece of automization software that, among thousands of other tasks, can monitor websites and send alerts.
Thanks to all for your prompt replies, I'm looking into some of the links now.
As for the comment about requiring a pay-for monitoring service for dependable stats, I am aware and plan to move to a more frequent monitoring system once we get going. However we are still in the startup/planning stages, and I just want to keep an eye on our server to ensure its as stable as it appears.