In the previous answers almost every one  stressed on the reasons why have the representations of monoids not been studied widely, which probably answers your question.

But I would like to mention a couple interesting cases were the structure of monoid-algebras are fully understood (over the field of complexes).


1.[Temperley-lieb Algebras](http://en.wikipedia.org/wiki/Temperley%E2%80%93Lieb_algebra), which are quotients of Hecke algebras.

2.Partition Algebras, which are invented and studied by Paul Martin.

Both of the above examples are part of a wider brach which is called Diagram Algebras.