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nl: number lines in existing file
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numbers each non-blank (by default) lines in a file.

$ nl -v STARTNUMBER -i INCREMENT -s SEPERATOR -w WIDTH -b STYLE -f FORMAT InputFile

All parameters are optional

STARTNUMBER: starting number default [1]
INCREMENT: increment [1]
WIDTH: number of columns for number [6]
SEPARATOR: string which separates number from the rest of the line [tab]
FORMAT: 
  ln (left justified)
  rn (right justified, leading blanks)  [default]
  rz (right justified, leading zeros)
STYLE:
  a number all lines
  n number no lines
  t number only non-empty lines
  pBRE number lines that match a basic regular expression

$ cat HelloKitty
Hello
My
Name is 
Kitty
Honestly Kitty!




$ nl HelloKitty
     1  Hello
     2  My
     3  Name is
     4  Kitty
     5  Honestly Kitty!

$ nl -i 3 HelloKitty
     1	Hello
     4	My
     7	Name is 
    10	Kitty
    13	Honestly Kitty!

$ nl -s ". " HelloKitty
     1. Hello
     2. My
     3. Name is 
     4. Kitty
     5. Honestly Kitty!

$ nl -bp"Kitty" HelloKitty
      	Hello
      	My
      	Name is 
     1	Kitty
     2	Honestly Kitty!

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For fun
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$ sed '=' infile | xargs -n 2            #using sed
$ awk '{print NR, $0}' infile            #using awk
$ grep -n ".*" infile  | sed 's/:/ /'    #using grep