BackgrounDRb, by Ezra Zygmuntowicz, is a system that lets you run long running tasks in the background, and not in the scope of your application. For example, you might have a Rails application that needs to send 100 e-mails, but rather than tie it to an HTTP request, you can simply pass off the job to a BackgrounDRb object and let the user know everything is proceeding as planned. There's more information here.

It's powerful stuff, and worth looking into if you have anything like mass mailing or other remote communications that isn't request dependent, but still needs to be carried out.