iD has released the source code for DOOM 3 BFG edition. If you are interested you can download it here:
https://github.com/id-Software/DOOM-3-BFG

That should make modding it easy, right?

HURRAY! :D

An interesting twist to the ongoing id/Zenimax/Rage/Doom 3 saga...

Interesting that they should release id Tech 5 code. I expect it's because it's a very heavily scaled down version of id Tech 5, but I wonder if they'll be releasing the Rage source code ahead of schedule?

so what could i do with the source code?

Assuming you have the necessary assets (aka the full game), more or less anything you'd like.
You do need to be able to write and compile the code though, and while the compiling is (usually) easy, the writing isn't that encountered that often.

i don't have the game, but thanks for the info - i assume this is how mods for other popular games are made ?

Unfortunately, the games that have the source code made public are very very few, thus most mods have to resort to other methods. Some games have mod tools made avalable (TES for example), others have packages that can be extracted and modified (UFO:A? for example), while others have to result to weirder methods.
For an example of what is possible with source code access, check JA2 1.13 compared to JA2 1.12

again thanks for the info, really interesting. There are really some talented peeps about :)

It makes moding a lot easier, and in a way it is then done with the developers blessing.

Wow I was almost certain none of the big developers would ever do that again. Step in the good direction i think, even though i don't use a lot of mods...

The most interesting thing for concerning this news (for me at least) is that it means there is now an easy way to transform the Steam exclusive BFG edition into a DRM-free version... not that I was very interested in the BFG edition to begin with but it's still good to know.
Well Id is one of the few to do it relatively regularly so it's not really that big of a surprise.

Was any of the original Doom 3 engine code used in Doom 3 BFG? If it was, did it being released under the GPL cause this to be released sooner to keep compliance with the license?

Either way, this should make the game far more popular for mods. Also like mentioned, a "steam free" version can be done now also making the game DRM free.

I wish more game developers would do this.

Wonder how this will impact The Dark Project?