Each of the DROD games have a level editor.

User made holds can be found here:
DROD - http://forum.caravelgames.com/viewboard.php?BoardID=11

DROD RPG - http://forum.caravelgames.com/viewboard.php?BoardID=266

Don't forget the level editor in VVVVVV.

Incidentally, there is a free "make and play" edition of VVVVVV, which includes the level editor and custom levels, but not the main game.
http://distractionware.com/blog/category/vvvvvv-make-and-play/

Jagged Alliance 2: Unfinished Business comes with a level editor, and does have quite a bit of maps/campaigns as well.
Jagged Alliance 2 with the 1.13 mod makes use of the same level editor, and there were alternate maps and/or new campaigns (Urban Chaos and Deidrianna Lives for vanilla JA2, AIMNAS for 1.13) as well.

Warcraft 3 and Starcraft 2 has great map editors.
People have even made RPGs and other separate games using that editor. I vaguely remember a map in WC3, it was a Final Fantasy fangame with limit breaks, summons and all, you could even save your characters between play sessions.

Because you hadn't mentioned them yet. ;)

I take it those have level editors for multiplayer maps?

They have complete development suites ;-)

Anything you want to make for a UT game, from custom weapons and maps, up to and including total conversions, you can with the tools provided.

Kivi's Underworld from Soldak have an editor. You need to finish the game to open it.

Edit - and not sure if it matters, but CW3 have more than an level editor.It comes complete with its own scripting tool. You can change everything in the game, and there are examples of it has been used to make everything from a Simon Says game to a top-down shooter. All behaviour and graphics can be edited

Btw, I tried to categorize the titles in campaign editors and level editors, based on how powerful the editors are, mostly with regard to whether it's possible to create your own stories with it. However, I guess that comes down to very simple features, such as the option to add text to a level (maybe even just on the starting screen), and I don't know a lot of the suggested games or their editors, so often it's pure guesswork on my part.

Not sure if it really makes sense to keep them divided into different categories that way and on what criteria it should be based on. I just thought there's a difference between a game editor like NWN, or even something simple and limited but still open for creativity like Fight the Dragon on one side, and a level editor for something like MouseCraft on the other, which allows you to make short new puzzles within the rules of the game but not add a lot of creativity to it otherwise (text, new environments, new graphics, new sounds etc.). So if you have any ideas how this could be handled and which games should be in what category, I'd gladly listen!

E.g. would it make a little more sense to group them by game genre? In the sense that you'd more or less know what to expect from a RPG editor or strategy map editor or FPS map editor or puzzle game editor? Hm ... Categorizing is always kind of tricky, but without it one tends to lose the overview quickly. :/

Good links there, personally I would state it more along the lines of, these are the main modding sites:
Moddb (www.moddb.com)
Nexus (http://www.nexusmods.com/games/?)
...

And these are additional or more specific:
Infinity Engine (not "enhanced"):
Sorcerors Palace (http://www.sorcerers.net/
Gibberlings 3 (http://www.gibberlings3.net/)

Mount & Blade:
https://forums.taleworlds.com/
...

Otherwise you will repeat a lot which can be covered just by saying moddb or nexus.

When I t comes to amount of maps and customisation for CW3 , I think the right word to use would be "some"...

You can find some of them on the forums ( http://knucklecracker.com/forums/index.php?board=32.0) and there is also discussions on how to make them and such. There is also a thread on how to use the scripting (http://knucklecracker.com/forums/index.php?board=25.0)

Other games with editors that I can think of right now (and not mentioned, I think) :

Far Cry
Far Cry 2
Far Cry 3
Gateways
Hatred (yeah... I know....)
Octodad: Deadliest Catch
Painkiller
Painkiller: Resurrection
Rage
Rise of the Triad
Serious Sam 1 (Original and HD, First and Second encounter)
Serious Sam 2
Serious Sam 3
Shadowgrounds
Super Meat Boy
Toki Tori 2+
Tower Wars

Mount and Blade definitely has total conversion capability, not just 'adventure making'.
There are more than enough examples, if you possess the Google-fu skill.

At least when I was trying it, Battle for Wesnoth has some interesting mods, including some that drastically change the gameplay (like turning the game into an RPG).

Also:

Amnesia: Dark decent
The Bards Tale
Blood 2 (with the expansion)
Civilization 2
Civilization 3
Civilization 4
Crysis
Dink Smalwood
Dungeon Siege
Dungeon Siege 2
F.E.A.R.
Max Payne 1
Max Payne 2
Medal of Honor: Allied Assault
Rayman Gold
The Settlers: Rise Of An Empire
Spellforce

(I will stop now...)

Thanks! I've added them to the second list, if you say they have level editors. If someone knows of any outstanding total conversion mods, I could add them to the third list, too.