Professionally I use Ruby mostly for Rails/Sinatra applications or
writing integration / acceptance test suites for Java API's or PHP
modules on a site.
Personally I use Ruby for pretty much everything unless there's a
better tool of for the job in another language or Ruby doesn't run on
that platform.
Both Rails and Sinatra are great for creating web services or job
schedulers, don't really do enough network admin to start building my
own tools.
David