I usually use Excel, but I see that you're Microsoft-free so perhaps that isn't an option.
I believe that AppleWorks will do this, if you happen to have that.
Otherwise, a slick little UNIX utility could be written in no time to process this. It'd be all of one line of code for sed (plain-text file containing "s/,/<press tab here>/g" without quotes, invoke with "sed -f script.txt infile.txt > outfile.txt"), and a handful of lines for Perl.