I don't know about stats, but if you just want an easy way to keep an eye on traffic/load/errors/etc., "gl_tail.rb":http://www.fudgie.org is a pretty effective live visualization tool. I've used it before, and it really doesn't seem to make much of a performance impact on the server.
I launched our new website entirely built on Ruby on Rails just a few weeks ago and want to make sure I can keep a close eye on our 4 production servers. I figured what better way then a bunch of led displays.
I'm not one to blog or even explain things very well, but I tried with the help of some photos here: http://www.82smugglers.com/blog/?p=7
For the most part this was put together just for fun, and now is pretty much eye candy as a week or two of getting to know rails in production mode had enabled me to tweak the servers to where they are fairly solid. (And I have god watching over my mongrels)
And for a question, does anyone know how to get easy stats out of rails or mongrel? I know I could write something to parse the logs, but that may take some resources doing it every 30 seconds to a minute.