automatic continuous backup hard drive

Ok, so this is what i got going on..

I have 2x 500gb identical HD. What i would like to do is use 1 of the 500gb hd as the main backup and the 2nd hd to continually backup the the 1st HD. RAID and Mirroring are not an option due to no controller card..

What kind of software would i need to buy to do this?