Personal favorites, there is mucho other great stuff out there ;
Mods: Monster Madness (my fave, all the Unreal monsters for you and the bots to frag)), Infiltration (real life weapons), Unreal4ever (lots of cool new weapons, including a wonderful freezer gun).
Maps: DM-Pallazo, DM-Foolsbravado, CTF-HallofGiants, AS-Bridge.
Skins: The "Brass" Boss skin, PredatorFX
Model: Darth Maul
Voice Packs: Bill Hicks, Black Knight (Monty Python)
As I said these are PERSONAL favorites - no doubt others will want to list theirs !