glass fiber
Fine threads of ordinary glass, drawn so thin (about 10 to 20 micrometres) that they bend without snapping and can be woven into cloth or chopped into mats. Set in resin they make fiberglass: boat hulls, bathtubs, wind-blade shells, circuit boards, and pink insulation batts.
Molten glass is pulled through tiny holes and cooled fast into thin filaments; being thin, each has few large flaws, so it reaches high strength (about 2 to 3.5 GPa) even though bulk glass is weak and brittle. Modulus is about 70 GPa (about the same as aluminum, roughly a third of carbon fiber) and density about 2.5 g/cm^3. Common grades are E-glass (cheap, electrically insulating, the workhorse) and S-glass (stronger, pricier).
Glass fiber is the cheap, tough, corrosion-proof, electrically insulating fiber that made composites mainstream. It gives good strength-to-weight, but its modest stiffness (much less than carbon) means glass composites are strong yet relatively flexible, great for boats and tanks, less ideal where you need extreme stiffness for the weight.
A fiberglass boat hull is E-glass cloth in polyester resin: it shrugs off seawater that would rust steel, takes complex moulded shapes cheaply, and flexes without cracking.
Cheap, tough, corrosion-proof, and insulating, but not as stiff as carbon.
Glass fiber is strong but not stiff: its roughly 70 GPa modulus is far below carbon's roughly 230 GPa, so fiberglass parts deflect much more for the same load. Cheapness and toughness, not stiffness, are its selling points.