Operating System
Boy, things have come a long way since I first installed SLS Linux on my pc using floppy discs back in 1994 
These days I prefer Ubuntu 6.06 LTS server edition. It is supported until 2011 - nice that you don’t have to reinstall the server in a year or two. Furthermore, deprec makes it extremely simple to setup a Rails stack on an Ubuntu server.
Webserver
nginx. Nginx is fast, stable, lightweight and has easy configuration.
Load balancer
Usuyally I use the one built in to nginx.
Alternatives: Pen or HAProxy.
Rails-server
Well, Mongrel of course. In a cluster handled by Mongrel cluster.
Database
MySQL. It’s the database used by most Rails applications. Furthermore, it has some nice scaling support with master-slave setups. A couple of years ago I used and liked PostgreSQL a lot. I still prefer PostgreSQL’s query analyzer to MySQL’s.
