Server Help Needed!

I'm probably going to be banned shortly from a server that I like.
I've spent a long time building on it and therefore would not like to lose the world.
I have heard of a program that allows you to save a world of a server and access it in single player but I don't know what it is called.
Can someone please tell me?

There's a cookie in it for you!


Please answer ASAP!
A link would help!