Reverse stdin and place on stdout

Bash - 7

tac|rev

tac reverses line order, while rev reverses character order.

BrainFuck, 10 characters

,[>,]<[.<]

Beats a good amount of answers for such a simple language.

Golfscript - 3 chars

-1%

obfuscated version is also 3 chars

0(%

here is an explanation of how % works