base64 and ROT13

base64

base64 PLAINTEXTFILE        # encode a file to base64
base64 -d BASE64FILE        # decode a base64 file
echo "STRING" | base64      # encode a string

ROT13

tr maps each letter 13 places forward, which both encodes and decodes.

cat FILE | tr '[A-Za-z]' '[N-ZA-Mn-za-m]'      # ROT13 a file
echo "STRING" | tr '[A-Za-z]' '[N-ZA-Mn-za-m]' # ROT13 a string