Continuation of the answer of Benjamin Steinberg; this paper gives a list of such known maps.
On the structure of entanglement witnesses and new class of positive indecomposable maps: by Dariusz Chruscinski and Andrzej Kossakowski; http://xxx.imsc.res.in/abs/quant-ph/0606211v1 and the journal version (if you want) is here.
It is slightly outdated but gives a good starting point. Not many classes of such maps are known in literature.
