Like crono said: HL2, most likely when you get sick of op4 HL then HL2 will be out (prays). But if your into modeling, and using a powerful engine there is always UT. But be warned, mapping for UT is very different from HL, because you delete space, and add "props" or models in this case. (excuse me if im using F***ing with UT's basics, Ive only seen the editor)

Quake III is good also, because the editor uses some of the same basic principals as hammer (csg bsp vis rad), only its better, because it is very hard to get a invalid brush (because the editor cuts it up for you into many brushes)

And as for Halo, there is an editor out, but I think white legion got a hacked version. But you'll probably need a high end computer to play test the map (as you need to even play the game).
note: please excuse my lack of "know how" on the other editors.

Ahem, let me correct some things in OmegaSlayers post:

How is CSG, Rad, BSP, and VIS got anything to do with the setup and controls of an editor? Nothing, thats what. Sure, they are there, but that doesnt mean they are exactly like Half-Lifes.

There already is a Halo editor out, its called: 3DS Max (Or Maya). Basically, you can model Halo levels already, too bad you will have to wait for an actual editor to add entites to the map. And no, your friend doesnt have a "h4xed" editor...There are some editors, but I wouldnt call them editors, since you can only add a few items, remove things and such nature. An editor lets you CREATE brushwork and architecture, while a "point and click" program lets you make 50 unwanted warthogs strewed across BloodGulch.

Unreal Tournament is a powerful editor, yes. But unless you actually like the game, I wouldnt bother. Whats the point in doing something you hate? Thats like going home and studying math...*In my case*. If you actually like Unreal's gameplay, you'll be happy to know its backed up by a quite powerful engine..So long as you have patient to learn how to Learn the Editor, Learn how to make Models, and most likly learn how to make textures, you'll be in the Unreal Mapping biz (Yes I know you dont NEED to learn to make models, but whats the point in not? Unless you have a modelling slave, your map will consist of NOTHING but generic static mesh's, which will haunt you down the road in UT mapping).

Also, if you dont want to learn an engine with scripting involved, your a f**king idiot. I mean, Half-Life relies heavly on scripted sequences, and so does Opposing Force. Scripting is a very powerful tool, IF you use it right. Dont try to run from scripting, it'll catch up with you later on in mapping...(Like me and that damn Vertex Manipulation!!)