Fiveruns offers a way to monitor the health of Rails systems. Their newest version (rewritten in Ruby from Java) is in beta, so it's free for the time being.

 After signing up, you're provided with a script that runs as a daemon on your server. It pokes around your system and discovers subsystems that can be monitored, like MySQL, OS health, Apache, Mongrel, etc.

 http://skitch.com/terrbear/mye8/subsystems

Once it's hooked up to those subsystems, you can see reports on how your server is running, like downtime in certain subsystems or resource consumption. You can also inspect specific methods to see how long they're taking to process, or where most of the execution time is taking place (M, V, or C). 

http://skitch.com/terrbear/myeu/times 

Further, you can have fiveruns start a notification chain when something bad happens, like if all mongrels went down. 

http://skitch.com/terrbear/myea/triggers 

Right now they'll let you know if your database is choking on a query, but they don't offer any recommendations. That's in their roadmap, where eventually the product should be able to say, "hey, you forgot to index this table. Why don't you do that now?" 

I mostly like it because it provides a good bird's eye view for predicting where you're going to need to grow next. At my previous post, once we loaded Fiveruns up, we saw that we had <2% of memory free during normal operation, and the next day there was a demo that crashed the application (no memory for new processes).