Subset sum, pseudo-polynomial time dynamic programming solution?
Rephrasing Kaveh's comment as an answer:

If you bound the size the values in the input (bound the number of bits for each value to be logarithmic in the total number of bits of the input) then the problem can be solve in polynomial time using dynamic programming. If they are not bounded they can have values which are exponentially large and the size of the table for the dynamic programming would be exponential.