On Mac OS X and Linux, you can convert files encoding in batch with iconv.

Check encoding of a file

Mac OS X: file -I file.txt

Linux: file -i file.txt

Convert files to new encoding

For example, to convert all .html files from ISO-8859-1 to UTF-8, and save them with the same name.

On Mac OS X:

find . -name "*.html" \
-exec sh -c "iconv -f ISO-8859-1 -t UTF-8 {} > {}.utf8" \; \
-exec mv "{}".utf8 "{}" \;

On Linux:

find . -name "*.html" \
-exec sh -c "iconv -f ISO-8859-1 -t UTF-8 {} -o {}" \;