DVORAK Keyboard layout

C - 144 characters

main(c){putch((c=getch())>33?c:"_#$%&-()*}w[vz0123456789SsW]VZ@AXJE>UIDCHTNMBRL\"POYGK<QF:/\\
=^{`axje.uidchtnmbrl'poygk,qf?|+~"[c-34]);main(0);}

Shell: Unix tr(1), 94

tr \''"+-/:-[]_b-{}' "-_}w[vzSsW]VZ@AXJE>UIDCHTNMBRL\"POYGK<QF:/={xje.uidchtnmbrl'poygk,qf;?+"

This command takes QWERTY on stdin and outputs DVORAK on stdout.

C#, 360 characters

Probably not the shortest, but it does exactly what you ask:

using System;class A{static void Main(){string[] q={"-=qwertyuiop[]sdfghjkl;'zxcvbnm,./","_+QWERTYUIOP{}SDFGHJKL:\"ZXCVBNM<>?","[]',.pyfgcrl/=oeuidhtns-;qjkxbmwvz","{}\"<>PYFGCRL?+OEUIDHTNS_:QJKXBMWVZ"};while(true){var c=Console.ReadKey(true);var a=c.KeyChar;int i,w=c.Modifiers==ConsoleModifiers.Shift?1:0;Console.Write((i=q[w].IndexOf(a))>-1?q[w+2][i]:a);}}}

If you press a key on your QWERTY keyboard, then the correct DVORAK character appears in the console.