First, the UDN has most of the built-in UE3 commands (there are a few minor ones you might stumble across in native code). See Unreal Engine 3 Console Commands, Unreal Editor Console Commands, and Stat Command Descriptions. Some built-in commands are included on this page, but not all.
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Shows server's current mod
Getall aocgri servername
Shows server's name
Debugging
showdebug
Enables the debug HUD (for programmers, search for the DisplayDebug function to see how to add information to this).
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Starts/stops stats gathering for Stats Viewer. Includes the same functionality as profilegame, and more. You can even connect Stats Viewer to a live server, although this will slow it down to almost unbearable levels.
Admin
admin
Followed by any command executes that command on the server. You can start profiling commands on the server without having to RDP in and use UnrealConsole by using this.
adminlogin
Followed by the server's admin password, allows you to log into a server as an admin.
adminlogout
Logs out as admin
getinfo
Gives a user's IP address and NetID (uniquely identifies them using their Steam ID), which is useful in case we want to globally ban someone (that is, put his IP/NetID in the config files of every server). Parameter is a string in the player's name, or leave the parameter out altogether to get info on everyone.
adminkick
Kicks a user. Requires you to type their whole name right now, which kind of sucks. It's a built-in command.
adminkickban
Kicks a user and permanently bans them (i.e. puts their IP/NetID in the config file). Also a built-in command.
adminchangemap
Changes to a different map. A check is performed to be sure the map exists.
admin servertravel
Changes to a different map without a "does exist" check.
AdminRestartMap
Restarts the map.
AdminChangeTeam
Swaps a selected player to the opposite team.
AdminChangeTeamDamageAmount
Change team damage amount on-the-fly; from 0.0 (no damage) to 1.0 (full damage)
ShowDebug
Not strictly an admin command, but for admins it shows extra information (server tick rate, auto-adjusted net relevancy bonus)
aoc_slomo
Change the server's speed to the string argument (from 0.001 to 2.0; that is, 0.1% to 200% speed)
pause
Pauses the server if the server has admin pausing enabled
aoc_endGame
Ends the current match.
Voting
votekick
Shows a list of all users matching the string argument (or all users if no argument is entered), and allows the user to pick one to initiate a vote kick against. Vote kicks can be adjusted / disabled in the config file.
votechangemap
Votes to change to a certain map.
voteresetmap
Votes to change to reset the current map.
Misc.
switchtoobservermode 2
Smooth spectator camera with all pawns always relevant (i.e. no pawns disappear because they're too far away and the server stops replicating them)
setobs2
Enter setobs2 to see all the setobs2 commands. For setting observermode2 settings (speed, acceleration, etc.)
colorblindsim
Simulates what a color blind player would see when playing the game (does not affect the Scaleform GUI). Enter without any arguments to get an explanation on its usage.
maplist
Shows all available maps matching the string argument (or all maps if no argument entered). This is currently hardcoded, so it might not be accurate.
disableallscreenmessages
Disabled "Paths needs to be rebuilt" and other similar level-related messages. May be useful for screenshots.
enablevignette
Argument of 0 disables it, 1 enables it. Turn it off for tiled screenshots.
togglehud
Argument of 0 disables the HUD, 1 enables it. Turn it off for fancy or tiled screenshots.
tiledshot A B
For super high-resolution screenshots. Use enablevignette 0 and togglehud 0 first (see above). Takes a screenshot of size A*ScreenResolution.X by A*ScreenResolution.Y, with B pixels of overlap. See http://udn.epicgames.com/Three/TakingScreenshots.html