Metallic Nanoparticles have attracted increasing interest due to their abilities to confine and manipulate light at the nanoscale via the excitation of surface plasmons, the collective oscillation of conduction band electrons. Surface plasmons can focus electromagnetic field into a subwavelength dimension and sense the change of the local dielectric environment, promising properties for surface enhanced spectroscopy and sensing applications. New interesting properties emerge, such as the Fano resonance, when clusters of nanoparticles are brought into close proximities. The reduced light scattering within the Fano resonance corresponds to the intense local fields around and within the clusters, a promising feature for the development of ultrasensitive chemical sensors. Cluster of nanoparticles also support a new plasmon resonance known as the charge transfer plasmon (CTP) when their junctions are made conductive. This thesis will focus on exploring new properties of complex plasmonic nanoclusters and applying them in applications of optical nanocircuits, frequency modulation, and surface enhanced Raman scattering. First, this thesis demonstrates the realization of 3D optical nanocircuits using plasmonic dimer antenna composed of two Au nanodisks separated by a gap. Individual antennas are loaded with media of specific geometries and dielectric properties, acting as optical nanocircuits that tune the resonance of the nanoantennas at visible wavelengths. Series and parallel combinations of nanocircuit elements (nanocapacitors, nanoinductors and nanoresistors) can be realized by appropriately loading specific arrangements of dielectric, semiconducting and metallic nanoparticles in the antenna gap. Second, this thesis investigates the CTP in nanowire-bridged dimer nanoantennas. The CTP arises at lower energies and depends sensitively on the junction conductance, offering a new route for achieving tunable plasmon resonances by modifying junction geometries or materials. Third, this thesis examines the complex near field properties of the Fano resonant plasmonic nanoclusters using the surface enhanced Raman scattering (SRES) both from molecules distributed randomly on the structure and from carbon nanoparticles deposited at specific locations within the structure. It is found that the largest SERS enhancement is achieved when the Fano resonance overlaps with the laser excitation wavelength and the specific stokes mode of the analyte. Finally, the plasmonic properties of the Fano nanoclusters are shown to be substantially modified by the addition of carbon nanoparticles. The placement of several carbon nanoparticles in junctions between multiple adjacent Au particles introduces a collective magnetic plasmon mode into the existing Fano dip, giving rise to an additional subradiant mode in the metallodielectric nanocluster.