I'm also going to watch for ping times to see when they happen. Since you only get 30 minute checks for free, I'm going to try to figure out if I can sign up for a few separate services, and if they will stagger the timing a bit.
If it's on your own server you could try Monit. Free open source linux software for monitoring basically anything that runs on your server or others. You can monitor websites that you don't even own, it just checks them at an interval that you set.
Just an alternative to the above really.
NB: Works well with a fantastic site called 'PagerDuty', now paid (used to be free) supports SMS and Phone call updates etc.
It doesn't just ping your server to make sure that its alive. It allows you to specify certain content/keywords that you want to monitor and it will send you alert email if the content/keywords are missing from your site.
Its designed for non-technical business users and it's very easy to use. Check it out!
As far as I know, host-tracker and pingdom are the best ones for website monitoring (if you want to monitor is your website online or offline), although Google Analytics also have an option where you can see is your website up or down..
Also for monitoring your website it is good to use statcounter (I use that), they are really good, as good as Google Analytics..
Try 100pulse.com - Free website monitoring service, monitors your site uptime/downtime with 15 mins interval, instant email alerts, customizable templates and much more added features.. the best of its kind I used to monitor my sites.