Quinolones are among the most common frameworks present in the bioactive molecules and hence represent an attractive starting point for the design of combinatorial libraries. Since 1962 4-quinolone-3-carboxylic acid derivatives are clinically used as antibacterial agents worldwide. Currently, fluoroquinolones are approved by the WHO as second-line drugs to treat tuberculosis (TB), and their use in multidrug-resistant (MDR)-TB is increasing due to the fact that they have a broad and potent spectrum of activity and can be administered orally. In the last years, quinolones endowed with "non-classical" biological activities, such as antitumor, anti-HIV-1 integrase, cannabinoid receptor 2 agonist/antagonist activities, have been reported by our research group as well as by other researchers. This review focuses on the 4-quinolone-3-carboxylic acid motif as a privileged structure in medicinal chemistry for obtaining new compounds possessing antibacterial, antitumor, anti-HIV, and cannabinoid receptors modulating activities. Synthetic approaches, structure-activity relationships, mechanisms of action, and therapeutic potentials of these novel classes of pharmacologically active compounds are presented.