We're currently a customer of AlertSite.com and use their
Web Site Performance Monitoring service to keep track of
whether our Web site on different servers are up or down.

I believe they use some type of ping test to provide this
type of information. We currently spend about $5 per month
per site to monitor once per hour from one location.

In recent months, we've run into some reliability issues and
are actively seeking alternative vendor solutions.

With this in mind:

1. Who are the top 3 competitors to AlertSite for this type
of service?

2. What are the pros and cons of each?

take a look at http://mon.itor.us - mon.itor.us is a free website
monitoring service. They have cool Web2.0 customizable user interface.