  Tuna fish.   
All fish contains mercury, and the larger, older, and fattier the fish is, the more mercury it's likely to have.  Canned albacore tuna is big and old, though not terribly fatty, so it likely has an intermediate amount of mercury.  Whether you think this is a problem depends on whether you trust the fish industry or the government less. 
Please note (especially if you are a StarKist corporate lawyer) that I am not singling out this particular brand of tuna fish as having more mercury than any other, it's just what I found at my local store.  In fact, it's the kind I eat quite a bit of. 
Source: Walmart 
Contributor: Theodore Gray 
Acquired: 8 February, 2009 
Text Updated: 8 February, 2009 
Price: $2 
Size: 4" 
Purity: <0.001%
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