• Ruby

'1000'.reverse.scan(/.{1,3}/).join(',').reverse

• Ruby

'1000'.gsub(/\B(?=(...)*\b)/, ',')

• Ruby

1000.to_s(:delimited)

• Rust

println!("{}", 1000.separated_string());

• C

setlocale(LC_ALL, "");
printf("%'d\n", 1000);

• C#

$"{1000:n}"

• D

string f = "%3,d".format(123456);

• Go

p := message.NewPrinter(language.English)
s := p.Sprintf("%d\n", 1000)

• Haskell

s :: Int -> String
s = intersperseN ',' 3 . show

intersperseN :: a -> Int -> [a] -> [a]
intersperseN x n = uncurry (<>) . foldr alg ([], [])
  where
    alg a (buf', acc)
      | length buf >= n = ([], (x:buf) <> acc)
      | otherwise = (buf, acc)
      where buf = a:buf'

• JS

new Intl.NumberFormat().format(1000);

• Java

NumberFormat f = new DecimalFormat("#,###");
String s = f.format(1_234);

• Java

String.format("%,d", 1000000);

• Java

DecimalFormat f = (DecimalFormat) getNumberInstance();
f.setGroupingSize(3);
String s = f.format(1_000);

• Java

out.printf("%,d", new BigInteger("1234"));

• Java

out.printf("%,f", new BigDecimal("1234.5"));

• Java

out.printf("%,f", 1_234.5);

• PHP

echo number_format(1000);

• Pascal

writeln(format('%.0n',[double(10000)]));   

• Perl

 sub commify {
    local $_  = shift;
    1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
    return $_;
}

• Python

format(1000, ',')

• Python

'{:,}'.format(1000)

• Python

f'{1000:,}'

• Scala

"%,d".format(n)