During the game press ~ to display the console, then type:

g_fly 1 - fly mode on
g_fly 0 - fly mode off
fly - fly mode
gotoroom roomXX- select room UU(for example gotoroom room01)
cl_showinfo 1 - show FPS on
cl_showinfo 0 - show FPS off
hitpoints <number> - set hit points
g_noclip 1 - clip though wall (if fly mode is on)
game_godmode <number> - god mode on, <number> - current health value
game_godmode 0 - god mode off
addwpn ? - weapons list
addwpn <number> - add weapon number from weapons list or full reload weapon
suicide - self suicide
save <filename> - save game with name <filename>
load <filename> - load game with name <filename>
map <filename> - load map <filename> (see map list in dir Areas)
exit - fast exit to Windows

U.S. Version Cheat Codes:
The following codes work with the U.S. retail version of the game. 

g_debugcmdproc_flag 
g_sptimer 
g_autosave 
g_stickyclient 
g_dontrewriteconfig 
g_developer 
g_printgroundnormal 
g_loadmap 
g_spectarors_only 
g_noclip 1 - go throu walls 
g_fly 1 - fly mode on 
g_fly 0 - fly mode off 
g_userconfig 
g_writedemotofig 
g_superman 
g_configname 
g_paused 
game_godmode 100 - gives health to 100 
game_godmode 0 - god mode off 
game_levelresult 
gotoroom room01 - goto room01 room02 etc.. 
w_showaxis 
w_lodend 
w_lodstart 
w_nolod 
snd_on 
changemode 
say 
shutdown 
ping_servers 
connect 
cl_showinfo 1 - FPS on 
cl_showinfo 0 - FPS off 
begin 
loadall 
detchbaselines 
fetchsound 
fetcjmodel 
fetchdata 
fetchvars 
fly - you fly 
disconnect 
disconnect_this_client 
new 
svmap 
load 
save 
suicide 
nextmap 
net_stats_reset 
net_stats 
maintenance_pwd 
changemode 
map - (mapname) 
demo_stop 
demo_play 
demo_record 
demo_record_stop 
show_sounds 
strictalias 
delayqueue 
dumpvars 
hitpoints 10000 
sbind 
delvar 
addvar 
addwpn ? - gives weapon list 
addwpn . - where . is add weapon number 
unbindall 
print 
exit 
list 
set