Three new series of thiazoles, quinolones, and thiazolidinones merged with benzimidazole, benzoxazole, and benzothiazole nuclei were synthesized. The compounds were subjected to Infrared, H-1 nuclear magnetic resonance, C-13 nuclear magnetic resonance, mass spectrometry, and elemental analyses. Cytotoxic activity of compounds were evaluated against breast (MCF-7) and colon (HCT-116) cell lines. Seven of the compounds had IC50 values between 0.0112 and 0.0198 A mu M and are hence more potent than the reference drug doxorubicin (IC50 = 0.0084 and 0.0088 A mu M against MCF-7 and HCT-116, respectively).