I've created a really simple ruby script to check whether my server is up or not. Here is pinger repository, it may exist a better and more complete solution but I like to hack my own and let it be so simple. If you need something as simple as that, use it or fork it and add any additional feature you may want.
This script is only for checking if your server is down. For other kind of monitoring I use monit, a service monitoring tool which I love and use. I also use exceptional notification to my rails apps for getting any errors. I use the pinger to check how often my server is down because the other logging tools can't tell you that, you have to ping the server from another machine to be sure.