My left eye/eyelid is considerably darker than my right. Is there anything I can do? (photo)

Unfortunately your picture is not very clear and it's hard to tell for sure, but I would tend to also comment that there is a little eyelid retraction on your left side, and not on your right.  Better photos might help us answer you more clearly.  

It is difficult to tell from the picture what the cause of this difference comes from, but it is usually due to either venous congestion or skin pigmentation. Sometimes a transconjunctival blepharoplasty (removing fat from an incision on the inner surface of the eyelid) can diminish the amount of venous congestion but this is quite variable. If it is pigment, then bleaching skin creams or laser resurfacing can be tried. Unfortunately, this can be a very difficult problem to resolve, especially since it is different on one side.

It appears that you have ptosis of the left upper lid, and therefore, you are seeing more of the skin between the crease and the eyelashes (pre-tarsal skin). This skin is thinner than the skin above the crease and may appear darker because the vessels are visible beneath. Other factors that can lead to dark skin around the eyes are allergies and chronic eyelid rubbing which cause inflammation and crepe-like texture. Decreasing hand to eye contact helps maintain healthy eyelid skin and normal anatomy. A consultation with an oculoplastic surgeon would be helpful for your ptosis. Best wishes.