I did not write this, I found it here.
1 - Open Terminal.app, type cd and then drag and drop the Folder containing the files to be renamed into the window.
2 - To confirm you're in the correct directory, type ls and hit enter.
3 - Paste this code and hit enter:
for f in *; do mv "$f" "$f.tmp"; mv "$f.tmp" "`echo $f | tr "[:upper:]" "[:lower:]"`"; done