You can try http://www.siteuptime.com/ http://www.hyperspin.com/ Both are quite good and they are free!
I second that, That is very good free service, I have used it for a long time on a couple of sites, I recommend that!
host tracker also offer upgrade options as well so you can simply use their free service (which is already awesome) or upgrade :0 plus not to mention HT gives many options in 1. You can also let your user do speed check instantly as well
Pingdom.com - they ping from 16 different locations and used by authority companies like Adbrite for example
www.host-tracker.com . They have a 30day free 1 min trial. I have the paid plan for 1 min lookups, its great
host-tracker.com is good , but hyperspin.com is beter ! in host-tracker you can create unreal uptime ... but in hyper i don't think so !
If you want to monitoring your website with SSL, you can try ipcheckpoint, they provide website monitoring service from serveral locations around the world. their website is www.ipcheckpoint.com