Mode for Begin: draw triangle strip.

If you want to draw something 2D in screen space then probably you will want to use GL.LoadOrtho or GL.LoadPixelMatrix.
If you want something in "3D", then probably you should consider using GL.LoadIdentity followed by GL.MultMatrix with the actual 3D transform you want your stuff to be in.

See Also: GL.Begin, GL.End.

	// Draws 2 triangles in the left side of the screen
	// that look like a square
	var mat : Material;
	function OnPostRender() {
		if (!mat) {
			Debug.LogError("Please Assign a material on the inspector");
			return;
		}
		GL.PushMatrix();
		mat.SetPass(0);
		GL.LoadOrtho();
		GL.Begin(GL.TRIANGLE_STRIP);
		GL.Color(Color(0,0,0,1));
		GL.Vertex3(0.25,0.5,0);
		GL.Vertex3(0,0.5,0);
		GL.Vertex3(0.25,0.25,0);
		GL.Vertex3(0,0.25,0);
		GL.End();
		GL.PopMatrix();
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    public Material mat;
    void OnPostRender() {
        if (!mat) {
            Debug.LogError("Please Assign a material on the inspector");
            return;
        }
        GL.PushMatrix();
        mat.SetPass(0);
        GL.LoadOrtho();
        GL.Begin(GL.TRIANGLE_STRIP);
        GL.Color(new Color(0, 0, 0, 1));
        GL.Vertex3(0.25F, 0.5F, 0);
        GL.Vertex3(0, 0.5F, 0);
        GL.Vertex3(0.25F, 0.25F, 0);
        GL.Vertex3(0, 0.25F, 0);
        GL.End();
        GL.PopMatrix();
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	public mat as Material

	def OnPostRender() as void:
		if not mat:
			Debug.LogError('Please Assign a material on the inspector')
			return
		GL.PushMatrix()
		mat.SetPass(0)
		GL.LoadOrtho()
		GL.Begin(GL.TRIANGLE_STRIP)
		GL.Color(Color(0, 0, 0, 1))
		GL.Vertex3(0.25F, 0.5F, 0)
		GL.Vertex3(0, 0.5F, 0)
		GL.Vertex3(0.25F, 0.25F, 0)
		GL.Vertex3(0, 0.25F, 0)
		GL.End()
		GL.PopMatrix()