This utility provided by yum-utils package complements the downloadonly plugin as it only works if the desired software is not installed. See how it fails and how it can be resolved with yumdownloader.
$ yum --downloadonly install openssh-server
Loading "downloadonly" plugin
Setting up Install Process
Setting up repositories
Reading repository metadata in from local files
Parsing package install arguments
Nothing to do
There is nothing to install, already installed, nothing is downloaded. Try now with yumdownloader.
$ yumdownloader openssh-server
openssh-server-4.3p2-19.f 100% |=========================| 252 kB 00:01
Done this, you have the rpm you needed.
Another frequently used option is to specify that you want to download the source rpms:
$ yumdownloader --source zsh
...
Enabling fedora-source repository
fedora-source/metalink | 1.7 kB 00:00
zsh-4.3.10-5.fc13.src.rpm | 2.6 MB 00:30
Note that yumdownloader has enabled the "source" repositories to be able to download.
