A leading abstract expressionist was
Jackson Pollock (1912-1956)
Began as a regionalist painter, student of Benton
Influenced by Mexican muralists (Orozco, Rivera, Siquieros); see his "Blue (Moby Dick)" 1943, with strong surrealist overtones
Developed "drip painting"
Employed by Works Projects Administration (WPA), one of the New Deal programs pioneered by Franklin Roosevelt to support artists during the Depression
Known for action painting, previously the style of Lee Krasner, Pollock's wife

a characteristic work made through drip painting, Pollock's "Full Fathom Five," 1947

"On the floor I am more at ease, I feel nearer, more a part of the painting, since this way I can walk around in it, work from the four sides and be literally 'in' the painting."
-- Jackson Pollock, 1947

"Full Fathom Five is one of the earliest masterpieces of Pollock's drip technique. The actual origins and initial development of this technique have never been fully explained, except by reading back from fuller photographic evidence produced about 1950, two or three years after this work was painted. Like other practical breakthroughs in twentieth-century painting, 'creative accident' seems likely to have played an important part, as Pollock probed and tested methods of paint application which promote the continuousness of line rather than the broken lines inevitable in the constant reloadings and readjustments of conventional brushwork. His solution was to pour from a can of domestic paint along a stick resting inside the container, so that a constant 'beam' of pigment came into contact with the canvas (which he left unstretched on the studio floor). The character of the line was determined by certain physical and material variables that could be combined in almost infinite permutations: the viscosity of the paint (controlled by thinning and dilution); the angle and hence speed of the pouring; and the dynamics of Pollock's bodily gestures, his sweep and rhythm, especially in the wrist, arm and shoulder. 'Like a seismograph', noted writer Wemer Haftmann 'the painting recorded the energies and states of the man who drew it.' In addition Pollock would flick, splatter and dab subsidiary colors on to the dominant linear configuration."
--Nicolas Pioch, http://www.ibiblio.org

Also see Pollock's "Lavender Mist: Number 1, 1950"

Other action painters

Willem de Kooning (1904-1997)
Also an action painter, though he created more recognizable images.
See some of de Kooning's work at the Artchive.

Mark Rothko (1903-1970)
known for color-field painting--painting with large fields of a single color.
See some of Rothko's work at the Artchive.

Robert Motherwell (1915-1991)
often combined color-field and action painting
See Motherwell's work at the Artchive.