Restrictions on VOC emissions have spurred the development of alternatives to oil-based polys. Consider the different characteristics of each type before you buy.
Oil-based. Turns slightly amber, which warms up a wood's color. Forms a hard, durable film in a few coats. Dries more slowly than water-based polys, so there's a longer wait between coats. Higher in VOCs than water-based finishes and therefore not available in all parts of the country. Cleans up with mineral spirits.
Water-based. Looks like milk in the can but dries crystal clear. Good if you're trying to preserve a wood's color. More watery than oil-based polys, so more coats are needed. Dries quickly. Lower in VOCs than oil-based finishes but still requires good ventilation during application. Cleans up with soap and water. Never use ammonia-based cleaners on the cured film.
Water-based oil-modified. Appears cloudy in the can but dries to a tough, amber-color film, like an oil. Fast drying and compatible with all woods; cleans up with water. Has the same low VOC levels as water-based polys.