Bleach: Lemon juice is an all-natural bleaching agent. Pour lemon juice on stains (test for colorfastness in a hidden spot first) and hang in the sun to dry, then machine wash as usual. For an allover white brightener, replace the bleach in your wash cycle with one cup of lemon juice.
Soak solution: For very soiled or perpetually dingy items (not just clothes—think towels, sheets, tablecloths), soak overnight in hot water with ½ cup lemon juice. Machine wash as usual.