For monitoring up to 25 sites WebGazer has a $15/mo plan. Have been happy with the service so far, and they've continued to add additional functionality over time to help the community monitor their services, including a cron job monitor feature which was added recently.
Simple, free tier, and provides some create tools.
Simple solution to this common problem. Works amazingly well and FREE.
Around the web
A sincere briefing about the re-launch of WebGazer.
Hyperping combines your uptime monitoring with a simple, beautiful and customisable status page. Get alerts and react before your users notice, communicate about an incident or a scheduled maintenance to your customers through email and SMS notifications!