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Do I put these in a mods folder? I'm confused...
Is Epic's official UT2004 Mega Bonus Pack included in GOG's instalation package?
Patejl: Is Epic's official UT2004 Mega Bonus Pack included in GOG's instalation package?

No, it's not. You just get UT2004 with all official patches applied.
You don't get the Mega Bonus Pack, you don't get the 64-bit patch and you don't get AS-Confexia, DM-Forbidden or the ONS Bonus Map Pack. However, you can get these online as they were freely released.
Note, if you do get the Mega Bonus Pack, the installer has a bug which changes some text data files to older versions. To correct this, after you've installed the Mega Bonus Pack, install the 32-bit v3369 patch.
Also worth a look is the Ballistic Weapons mod - on moddb:
The planetunreal links are broken.
In case anyone was wondering, I can confirm that the GoG UT2K4 ECE version does include Epic's Official Mega Bonus Map pack, which includes the 3 new vehicles, 6 new characters and assorted maps.
what mods are still active?
Wanted to mention that Deathball is now basically standalone (still free).

Thing is, steam account is required.
Post edited October 25, 2015 by Projectsonic
Ballistic Weapons is my jam. It contains various guns based on real counterparts with highly customizable settings (how recoil and inaccuracy work).
Reinstalled it after few years of pause only to be trapped by settings, there is like 3-4 way to configure which weapons are avaliable but only one worked. Weird. Just like guns.

BW+Seargant Weapon Pack = good times

Here's a pretty large pack of UT99 maps remade at 1.25 the original scale. Unlike past remakes over the years, the eye candy is extremely minimal and they're even more faithful to UT99 than past ones in general. This includes DOM maps, but they're made for the Classic Domination gametype (it's a mod for UT99-style domination) as opposed to Double Domination.