okay, let me see if i can understand this (because i have had the same question)
you have your root (/) partition, and next to it is an empty partition, correct?
you simply want to make one partition out of that?
well, the program i know that can do this is Partition Magic 8 for windows, and it's not free...
in PM, delete the unused partition and resize your root partition to take up that space
a program called qtparted for linux can do the same; is is a PM clone using linux native tools; make sure to read the Readme's and download the necessary external programs
another option (just though of it but is VERY dangerous); use fdisk to delete your unused partition and your current root partition; then create a new partition that starts in the exact same place as before and ends at the end of your unused partition; do this only as a last resort! backup before you even Consider this!