  We propose three-dimensional N=6 superconformal U(N) X U(M) and SU(N) X SU(N)
Chern-Simons gauge theories with two pairs of bifundamental chiral superfields
in the (N, M) and (\overline{N}, \overline{M}) representations and in the (N,
N) and (\overline{N}, \overline{N}) representations, respectively. We also
propose the superconformal U(1) X U(1) gauge theories that have n pairs of
bifundamental chiral superfields with U(1) X U(1) charges (\pm 1, \mp 1) or
(\pm 1, \pm 1). Although these U(1) X U(1) gauge theories have global symmetry
SU(2n), the R-symmetry is SO(6) for n=2, and might be SO(2n) or SO(2n+1) for 3
\leq n \leq 8. In addition, we show that from either the generalized ABJM
theories, or our U(N) X U(M) theories, or the N=5 superconformal O(N) X USp(2M)
gauge theories, we can derive all the N \geq 5 superconformal Chern-Simons
gauge theories except the N=5 superconformal G_2 X USp(2) gauge theory and our
U(1) X U(1) gauge theories with n \not= 2 and 4. Furthermore, we derive the
three-dimensional N=8 superconformal U(1) X U(1) gauge theory from the BLG
theory, and study the corresponding moduli space. With the novel Higgs
mechanism in the unitary gauge, we suggest that it describes a D2-brane and a
decoupled D2-brane.
