I've tried Hydroquinone pads and retinol creams. Hydroquinone is an expensive substance, but it certainly does wonders. Hydroquinone is apparently a carcinogen, so you should use it in moderation. Retinol is not expensive, but I am not a huge fan of retinol because it peels your skin.
I've also used lemon juice for skin bleaching and I think this is the most inexpensive treatment. Just apply lemon juice on the darkened areas and rinse after 15 minutes. Try this everyday for a week and you should see some progress.