The Reckoning by Xatrix or otherwise known as Mission Pack1, contains a few new weapons and a new power up. 
You need to have a retail copy of The Reckoning in order to play this mode.
You will also need 92mb of free hard drive space on the same drive as quake2.

If you have both of these.
Make a copy of the pak0.pak from the xatrix folder to the 3zb2 folder.
Rename this pak from pak0.pak to pak9.pak (this will prevent any mishaps when installing ctf support)

Singleplayer Maps :

xswamp "The Swamps"
xsewer1 "Sewers"
xsewer2 "Waste Sieve"
xcompnd1 "Outer Compound"
xcompnd2 "Inner Compound"
xreactor "Core Reactor"
xware "The Warehouse"
xintell "Intelligence Center"
industry "Industrial Facility"
outbase "Outer Base"
w_treat "Water Treatment Plant"
badlands "Badlands"
refinery "Refinery"
xhangar1 "Lower Hangars"
xhangar2 "The Hangars"
xship "Strogg Freighter"
xmoon1 "Cargo Bay"

xmoon2 "Command Center"

Deathmatch Maps :

xdm1 "Munitions Dump"
xdm2 "Deadly Reckoning"
xdm3 "Reservoir Stroggs"
xdm4 "The Sludge Pit"
xdm5 "Stomping Grounds"
xdm6 "Hidden Agenda"
xdm7 "The XEdge"

Don't forget you need the chn's for these maps
Bot authors site has all the Reckoning chn's.