One thing that would really make me go nuts is any Quake related stuff like maps, weapons, monsters, etc for Unreal Tournament.
Entirey possible, with the correct tools
Quake 1 assets can be ported easily with mdl2unr.
As for Quake 2, I've seen guides around on how to decompile or convert a MD2 file to Unreal 3D, but none worked
Quake 3 would be news to me. I've not seen any program or tool that can decompile an MD3 model. I can port the models to Unreal/UT, but there would be no animations.
Half Life and Counter Strike assets can be ported to Unreal/UT too. All you need is Milkshape, the MDL file and a LOT of patience
There are so many games that can have it's content ported to Unreal. Pretty much ANY game that uses the Unreal Engine (From KHG to UT3) can have something ripped from it and made functional in Unreal. I Half-ported the UT2003 weapons to Unreal once (Still got that source somewhere...) and some other UT2004 mods (Frag Ops and Tactical Ops 2 Weapons)