An almost trivial one-liner with curl can do this kind of service.
That is the figure you should be looking at because even if you assume only a single day of work you can already buy many months of pingdom service for the same money.
Moreover even the smallest monitoring utility takes quite a bit more effort than a single day once you take SMS alerts with backoff (you don't want to spam, right?), scheduled downtimes and overall babysitting into account. Been there, done that.
So, look at the big picture and those $10 bucks suddenly become a no-brainer.
You have to be using one of the hosts they cover (currently EC2, EC2 Europe, Rackspace, Slicehost.). They will probably add a bunch more soon, as they are organizers of the libcloud project, which aims to build interfaces for all popular providers. Code checked in so far covers Linode, vps.net, vCloud Express, for example. (http://libcloud.org/)
It's easy to install because you don't have to install any agents on your system. You just plug in your (provider) key at the friendly, easy Cloudkick dashboard, et voila. All the accuracy of agented monitoring, none of the mess/packages/server set-up.
(Lots of hosted monitoring services act like "agentless" monitoring, which means not that accurate or fast to notice things.)
Also, it's free. (Though it seems like they may have premium services one day.)
For the free service, I'm looking for something that would notify me by email and SMS if the site is down or slow (within about an hour). So far it looks like http://aremysitesup.com/premium/ can do this. Pingdom is way too expensive for my needs right now.
I would love to support a fellow Hacker News reader. Does your app fit my needs?
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.
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.)
* Offer very gradual price increases for increased service (gradual like this: http://site24x7.com/site24x7-pricing.html)
It sends you an email when a site goes down. The free version pings every 20 minutes or so.
I wrote a script, run via cron, that checked a ticket web page every 30 minutes for concert ticket availability. If the text changed, it implied that the tickets were available; the script sent an email to my phone, a text message, telling me call for tickets. ( Ratatat was the band )
Monit after a tip here, great little program.
I'm not familiar with that service btw, just remembered seeing that here a few days ago. I use nagios both at home and at work.
It requires some setup but the big advantage of this is that it can not only tell you when your site is down, but sometimes ACTUALLY FIX IT by restarting nginx or whatever you need it to do.
This is something a monitoring service can't do since they don't have access to your box. Of course it's not going to be able to fix all situations that could cause downtime, but if it hits a scenario you anticipated it will.
If a service crashes repeatedly then fix the service, don't deploy bandaids. Those bandaids tend to pile up and god help you when that pile comes crashing down...
Not only it monitors if your site is down, but also if it has been blacklisted, defaced, etc.
Also your site is lite on details - makes a lot of claims about protecting this or that without any explanation on how this protection is achieved.
How do you validate integrity of the content? Do you compute and compare hashes of all files?
Also, you should list the world-wide locations you are checking from.