Original Post
I´ve implemented rectangular area lights in glsl (I missed them from Maya and 3dsMax :__ ). They´re not the real thing but an approximation, however they work quite well. I´ve used a spotlight as a basis, so they´re very simple and fast, also. You can orient them and change their width and height. They cast no shadow, that will be a different thread :D (a very blurred pcss shadowmap should do the trick). I´m very excited about them because (at least for me) they open a new world of illumination schemes! Screenies: 
Code: (there is room for optimization but it´s usable as it is) Specular is correctly calculated as you can see, along with diffuse and ambient. I see lots of potential uses for this in any graphics application, for example: -fluorescent bulbs (spot, omni or directional can´t even approximate this) -windows -glowing panels -lava pits ... You can even fake GI (fake because you are emitting light, not reflecting it) using some well-positioned area lights: in a room that has bright green floor, placing a greenish area light pointing upwards there can give quite a boost to the scene at a negligible cost. [Edited by - ArKano22 on November 6, 2009 6:23:14 PM]
Code: (there is room for optimization but it´s usable as it is)
vec3 projectOnPlane(in vec3 p, in vec3 pc, in vec3 pn)
{
float distance = dot(pn, p-pc);
return p - distance*pn;
}
int sideOfPlane(in vec3 p, in vec3 pc, in vec3 pn){
if (dot(p-pc,pn)>=0.0) return 1; else return 0;
}
vec3 linePlaneIntersect(in vec3 lp, in vec3 lv, in vec3 pc, in vec3 pn){
return lp+lv*(dot(pn,pc-lp)/dot(pn,lv));
}
void areaLight(in int i, in vec3 N, in vec3 V, in float shininess,
inout vec4 ambient, inout vec4 diffuse, inout vec4 specular)
{
vec3 right = normalize(vec3(gl_ModelViewMatrix*gl_LightSource.ambient));
vec3 pnormal = normalize(gl_LightSource.spotDirection);
vec3 up = normalize(cross(right,pnormal));
//width and height of the area light:
float width = 1.0;
float height = 4.0;
//project onto plane and calculate direction from center to the projection.
vec3 projection = projectOnPlane(V,vec3(gl_LightSource.position.xyz),pnormal);// projection in plane
vec3 dir = projection-vec3(gl_LightSource.position.xyz);
//calculate distance from area:
vec2 diagonal = vec2(dot(dir,right),dot(dir,up));
vec2 nearest2D = vec2(clamp( diagonal.x,-width,width ),clamp( diagonal.y,-height,height));
vec3 nearestPointInside = vec3(gl_LightSource.position.xyz)+(right*nearest2D.x+up*nearest2D.y);
float dist = distance(V,nearestPointInside);//real distance to area rectangle
vec3 L = normalize(nearestPointInside - V);
float attenuation = calculateAttenuation(i, dist);
float nDotL = dot(pnormal,-L);
if (nDotL > 0.0 && sideOfPlane(V,vec3(gl_LightSource.position.xyz),pnormal) == 1) //looking at the plane
{
//shoot a ray to calculate specular:
vec3 R = reflect(normalize(-V), N);
vec3 E = linePlaneIntersect(V,R,vec3(gl_LightSource.position.xyz),pnormal);
float specAngle = dot(R,pnormal);
if (specAngle > 0.0){
vec3 dirSpec = E-vec3(gl_LightSource.position.xyz);
vec2 dirSpec2D = vec2(dot(dirSpec,right),dot(dirSpec,up));
vec2 nearestSpec2D = vec2(clamp( dirSpec2D.x,-width,width ),clamp( dirSpec2D.y,-height,height));
float specFactor = 1.0-clamp(length(nearestSpec2D-dirSpec2D)*shininess,0.0,1.0);
specular += gl_LightSource.specular * attenuation * specFactor * specAngle;
}
diffuse += gl_LightSource.diffuse * attenuation * nDotL;
}
ambient += gl_LightSource.ambient * attenuation;
}
