http://runslikeclockwork.com/
Very simple web monitoring system that checks if a site is online and sends alerts by SMS If down. They also offer a free monit aggregation service. Plans start £3/month basic, £5/month for 5 sites every 30 seconds plus 15 sms credits / month.
http://alertfox.com/
Monitoring with a focus on browser experience. Plans are €10/month for simple, $50/month for 1 browser, $100/month upwards for the full service.
