Auto Join is a minecraft mod that makes it easier to join public servers.
When disconnected from a server, the user is giving the option to Auto Join it.
Auto Join will ping the server until it sees that there is a slot open. If there isn't, it waits the specified delay and tries again.
If an error occurs, Auto Join will wait the specified delay until trying again (otherwise, you can get into some nasty exception loops).
Forum Thread: http://www.minecraftforum.net/topic/1922957-/