Yeah, nimbu.net does do what you're asking for and it will always be free for one site. You will get notifications by email and/or SMS and/or twitter within minutes of your site going down. The SMS option won't always be free, but you can send via Twitter and then get Twitter to SMS you for a free option.
Edit: I just re-read about having lots of little sites. I will consider a reduced-functionality (eg. as you say within the hour notification) but allowing more free sites as an option.
"always free for 1 site" seems like a bad call. unless you have a ridiculous number of sites, is it really that much work to make a different account for each site, either with different emails or using a gmail/google apps email and putting a different +whatever in your address? I would guess that your rate of paying customers will be pretty low.
You make a good point and I've thought about that a lot but competitive sites offer free accounts so it would be hard for me not to have a similar option.
You could do something like zoneedit and just have some stern warning about dropping your accounts or charging you for back usage if you get caught circumventing the free limit by opening multiple accounts.
aremysitesup.com is free for five sites, not one, so they are offering me a better solution for my particular problem. The next step up, $35/year, is also low enough to not drive me away. I also see that nimbu.net says, "At the moment nimbu is free to use with a limit of 3 websites." and I have more than 3 I want to monitor right now.
As you can see, you have a lot of sites competing with you (which means you have a market!). Now you need to distinguish yourself.
I'm having trouble finding a service that does the following:
* Offers me a free service that's suitable for sites that aren't that critical.
* Once I need a higher level of service, offers me a way to gradually turn up the service and payment.
Maybe I'm expecting too much out of a free service.
EDIT: I realize what my expectations are now. I think Google has spoiled me. Here's what I expect (and I think the web monitoring service that can offer this solution will likely win):
* Monitor an unlimited number of websites at a frequency of not more than 60 minutes. Will send an SMS and an email if the site is unavailable or too slow. (Maybe there is a cap on the number of SMS messages per month.)
Thanks for the detailed response. It is useful to hear real-life requirements and I'll bear this in mind when thinking about pricing options. I must admit though the number of options on site24x7 is a bit off-putting to me, I was thinking about much easier 'packages'
Edit: I just re-read about having lots of little sites. I will consider a reduced-functionality (eg. as you say within the hour notification) but allowing more free sites as an option.