The characteristics of a special type of guided wave, namely the surface plasmon-polariton (SPP), have been investigated. We focus on the backward wave propagation of SPPs in two specific plasmonic waveguides at optical frequencies. These plasmonic structures form a unique class of planar left-handed medium due to the single - epsilon material parameter. They are also able to achieve lateral confinement on the nanometer scale, which makes them attractive for nano-optics applications. Additionally, a contra-directional coupled-line coupler based on the guidance of SPPs has been proposed, designed and simulated. This coupler exhibits co-directional phase and contra-directional power flow in the two branches. The contra-directional coupled mode theory has also been derived to reveal the phenomenon of two eigenmodes with complex-conjugate propagation constants. This leads to a high coupling level, short coupling length, and high isolation. Finally, the effects of material losses and finite geometry have also been investigated.