If the paint can is rather full, you must first harden the liquid. Most home improvement and hardware stores sell a powered "paint hardener" product. The product is a powder that can be poured directly into latex paint and will harden up to 2/3 gallon. This product costs around $3 per packet. Once the liquid is hardened, replace the lid on the paint can, and throw everything away in the regular trash. (In Murrysville, Ferri's True Value Hardware in Town Square sells this paint hardener. Find it in the "Stains" aisle at the store.)
If the paint can is less than 1/2 full - pour liquid over an absorbent substance, such as clay-based cat litter or shredded newspaper. Set out in the sun to dry and throw everything away in the regular trash.
Oil-based paint is considered hazardous material and should be collected by Republic Services through their Household Hazardous Waste Program. This at-your-door service is free to Murrysville's residents on the curbside recycling program. Simply call Republic Services at 877-788-9400 (toll-free) to scheduled a collection.