Good program for synching two hard drives?

[Totality]

Is there a good free program out there that people use for synching an internal and external drive? Something that lets you select, for example, two folders and then hit sync and it will match the folders up copying files both ways?

I'm running windows 7 home premium if that includes such a tool, if not then I don't mind 3rd party ones.

 

[Totality]

Cheers. I saw synctoy but though it could only handle single folder merges, but looks like you set up "Folder Pairs" in a list so its all good! Installing now...