Generalizations
Automorphic forms generalize the idea of modular forms for general Lie groups; and Eisenstein series generalize in a similar fashion.
Defining OK to be the ring of integers of a totally real algebraic number field K, one then defines the Hilbert-Blumenthal modular group as PSL(2,OK). One can then associate an Eisenstein series to every cusp of the Hilbert-Blumenthal modular group.
Read more about this topic: Eisenstein Series