For excel 07 / 10
=IFERROR(INDEX($A$1:$A$7,SMALL(IF(ISERROR(MATCH($A$2:$A$7,$B$2:$B$7,0)),ROW($A$2:$A$7)),ROW(A1))),"")
For older versions, could probably be improved.
=IF(SUM(IF(ISERROR(MATCH($A$2:$A$7,$B$2:$B$7,0)),1))< ROW(A1),"",INDEX($A$1:$A$7,SMALL(IF(ISERROR(MATCH($A$2:$A$7,$B$2:$B$7,0)),ROW($A$2:$A$7)),ROW(A1))))
Confirm array with Shift Ctrl Enter for both versions.