I made my own realm list changer or patcher, as I like to call it, because I didn't know there where others readily available. That said, I think mine will be useful to some people because its in spanish, and I'll also translate it in the coming days, hopefully it will be available in many languages.
About Realm Patcher
Realm Patcher is a small tool to modify the realmlist.wtf file on your World of Warcraft installation path to play on private servers. It allows to define custom realm and patch URLs and to restore the original ones. When modifying the file you have the option to delete your World of Warcraft configuration and cache directories, in case you switch servers and want to start anew.
It can also work as a private server launcher. It should work for enUS, enGB, enMX and enES clients, but only enUS clients have been tested so far.