GL.LINES

説明

線(Line)描画の Begin モード

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.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    public Material mat;
    private Vector3 startVertex;
    private Vector3 mousePos;
    void Update() {
        mousePos = Input.mousePosition;
        if (Input.GetKeyDown(KeyCode.Space))
            startVertex = new Vector3(mousePos.x / Screen.width, mousePos.y / Screen.height, 0);
        
    }
    void OnPostRender() {
        if (!mat) {
            Debug.LogError("Please Assign a material on the inspector");
            return;
        }
        GL.PushMatrix();
        mat.SetPass(0);
        GL.LoadOrtho();
        GL.Begin(GL.LINES);
        GL.Color(Color.red);
        GL.Vertex(startVertex);
        GL.Vertex(new Vector3(mousePos.x / Screen.width, mousePos.y / Screen.height, 0));
        GL.End();
        GL.PopMatrix();
    }
    void Example() {
        startVertex = new Vector3(0, 0, 0);
    }
}