MultiSpawns allows you to have multiple spawn points set for your server and will randomly select one of these spawn points on each player join.
You can find the source code for this plugin on GitHub https://github.com/ConnorLinfoot/MultiSpawns/
I have a Jenkins server which is where any development builds will be, note these builds are NOT approved by Bukkit staff and are for testing only http://ci.connorlinfoot.com:8080/job/MultiSpawns/
The idea for this plugin came from a plugin request thread on Bukkit http://forums.bukkit.org/threads/multiple-spawns.324385/