sulfonamide
Some bacteria, unlike us, have to build a vitamin called folate from scratch in order to grow. The first sulfonamides worked as a disguise: they looked enough like one of folate's building blocks to fool the bacterial machinery, jamming it and starving the microbe of folate while leaving our cells (which get folate from food) unharmed.
Chemically, a sulfonamide contains the sulfonamide functional group, a sulfur atom double-bonded to two oxygens and single-bonded to a nitrogen (R-SO2-NH2 or its derivatives). The antibacterial sulfa drugs mimic para-aminobenzoic acid and competitively inhibit dihydropteroate synthase, an enzyme in the bacterial folate pathway; this is a classic case of selective toxicity.
Over time the sulfonamide group proved to be a versatile pharmacophore well beyond antibiotics. By chemists varying what is attached to it, the same group anchors thiazide and loop diuretics, carbonic anhydrase inhibitors, some antidiabetic sulfonylureas, and the COX-2 inhibitor celecoxib, because the SO2-N motif makes useful hydrogen bonds and can mimic phosphate or carboxylate groups.
An honest caveat: 'sulfa allergy' is a real and sometimes serious problem, most strongly associated with the antibacterial sulfonamides; cross-reactivity to non-antibiotic sulfonamides is generally much lower but is still considered case by case.
The same SO2-NH2 group that lets a sulfa antibiotic mimic para-aminobenzoic acid also lets acetazolamide grip the zinc of carbonic anhydrase, illustrating one functional group serving many targets.
One versatile functional group acting as a pharmacophore across many drug classes.
Prontosil, the first sulfonamide antibacterial, is itself inactive in the test tube and is in fact a prodrug: the body cleaves it to release the active sulfanilamide.