We use GlobalNetWatch (
www.globalnetwatch.com), primarily because they had an office locally and salespeople came to visit us after finding us online. They alert me via text message to my cell phone when something goes down, which fortunately has only been twice in the last 18 months. We were impressed with the number of data centers they used, number of major backbones, and the reporting features.
Be careful as some services just send a ping to monitor. That's a good indicator if your internet connection is up, but Apache crashed on our server once and pings still responded while nothing came up when you tried to go to our site. GNW tries to pull your homepage, which for us tests our database connection, Apache, and our ISP all at once.
Brian.