I'm surprised no one has mentioned it yet.
Gamestudio game development system
Its made by a German company, and it is a full featured game development kit, consisting of a level editor, a model editor, and a script editor.
Pros:
1. All in one game development kit
2. Fairly easy to use, I learned on my own with the manual.
3. Able to import models from other tools like 3DS Max
4. Features its own scripting language called Lite-C
5. Able to use C++ (I never tried this)
6. Comes with some basic game templates for an FPS Game
7. Features multiplayer capabilities
8. Has it's own Physics engine
9. Large community and forums - some free game downloads and demos
10. relatively low price (Standard is around $50, commercial $200, pro $1000)
11. free demo version (30 Days)
12. Lots of 3rd party add-ons
13. Supports real time shaders.
14. New features are added regularly
Cons:
1. It's a bit challenging to get shaders to work, they are not included by default, but then I'm not a programmer.
2. You need to be able to program in order to make a good game. The template games included are rather basic. If you just want to make models or levels, your better off with unreal or some other sdk.
3. The model editor it comes with is easy to use, but is missing a lot of features you might be used to with more professional software.
4. Sometimes the manual is referring to an older version of the software.
5. Multiplayer runs really slow when you have more than like 4 players (but third party solutions are available)
I've had the commercial version for several years, and I like it a lot.