For stats, I use Ganglia. The newest versions, which sadly have to be installed from source on most Linux variants, have support for Python modules. So, in addi

For stats, I use Ganglia. The newest versions, which sadly have to be installed from source on most Linux variants, have support for Python modules. So, in addition to the out of the box stuff, I monitor a whole bunch of other stuff related to how my applications are doing (signups, item counts, etc). For daemon monitoring and watchdog-ing, I use monit. It emails me if there are problems with the server, and can automatically restart processes if they fail or get out of control.