Enable Cheat Mode:
Press ~ to bring down the console. Now enter /spdevmap XXXXX

In place of XXXX, enter a map name such as q3dm1, q3dm17, or q3tourney2. The game will now load in cheat mode. During play, press ~ to bring down the console and enter the following:

/god                   God mode
/iamacheater           Unlocks all levels at Skill 1
/iamamonkey            Unlocks all levels at Skill 100
/noclip                No clipping
/model sarge/krusade   Extra Sarge Skin
/give all              Gives all weapons
/give XXXX             Give item XXXXX, from the list below:

allammo
armor
battle suit
bfg10k
flight
gauntlet
grapping hook
grenade launcher
haste
health
invisibility
lightning gun
machine gun
medkit
personal teleporter
plasma gun
quad damage
railgun
regeneration
rocket launcher
shotgun

Hints

Defeat Xzearo:
There is a glitch in his targeting system! He can rail you twice at any Distance or use his BFG to fry you, but, if you grab a railgun and move behind the pillar next to it...you can inch very slowly to the point where you can see the boost pad on the other side of the map without exposing your entire body. If you do it just right...You can score at least 3 frags on Xzearo. Unfortunately the computer allows him to respawn near you. If this happens...grab the newly spawned rail gun (so he can't) and run like hell to the boost pad. When you reach the far side...Hide behind the pillar and keep railing him as soon as he tries to use the boost pad after you. With luck, he won't spawn behind you much, and you will be able to pick him off enough to meet the frag limit. ENJOY!!

Thanks to Revolution readers Vipa, Marcel Maagdenberg, Geforce, Rampage99, Monzur Morshed, [PoW] CoolMan, Jonathan Norman, kyle bjalme, Candyman, Tom T. Chen, Earthquake, and Sanjeet S. Sidhu-Brar!