Simplest way to write output message to 'output window' in Visual Studio 2010?

For debugging purposes you could use _RPT macros.

For instance,

_RPT1( 0, "%d\n", my_int_value );

It only accepts a string as a parameter, not an integer. Try something like

sprintf(msgbuf, "My variable is %d\n", integerVariable);
OutputDebugString(msgbuf);

For more info take a look at http://www.unixwiz.net/techtips/outputdebugstring.html