As you will see, repositories can hold thousands of RPMS or just one or two. Depending on your needs, you can happily just install a large software repository, and install the other software by hand, but if you want easy upgrade options, add the small repositories, too.
RPMFusion - This repository is a combination of three different older repositories (FreshRPMs, Dribble, and Livna). It holds hundreds of different software packages, including free and nonfree packages. If you are only going to add one repository to your yum.conf file, this is it. The only downside that I can see is that they only support FC 10 and 11. This repository does have an .rpm based installer, just choose your version and install the .rpm file, and you're in business.
Skype Repository - This repository only holds the Skype software. It is useful, as the Skype software is not available elsewhere, but not really necessary if you're not going to install Skype. You will have to download the public key and install it and then edit your yum.conf file to add the repository.
Google Repository - This repository also only holds two files, Picasa and Google Desktop for Linux. Again, you install a public key then add the repository by hand.