Lots of things, here's a quick summary of some of the features
'That green message...'
By far the most popular asset of Admin Mod is the ability to send messages, to one or all players on the server, that will appear in different places on the screen allowing server advertisments, notices and other important information to be passed along without fear of losing the message in the chat window. The messages can be any colour you desire and can even fade from one shade to another
Votes, votes everywhere!
Using Admin Mod you can start votes (and optionally allow users to start votes) for multiple options. They can be used to pass important messages that will not disapear until the user votes, to give users the choice of whether or not to kick disruptive users or to enable weapon restrictions for the next map. Votes are also avilable through the plugin interface allowing as many custom votes as you can dream of.
HALFD style voting
HALFD is anoter server administration tool which allows users to vote by simply saying their choice. This ability is emulated in one of the default Admin mod plugins allowing more versatile user-server interactions.
Highly configurable per-user access
The main feature and strength of Admin mod is the abiltiy to configure per-user
access to commands. Rather than allowing all your Admin Remote console access
and having to change the password for everyone should it be compromised you can
give each user access to different functions with a password unique to them.
Should the password leak only one pass needs to be changed and life continues
as normal for your other Admins.
Each Admin will only be able to see the commmands they have access to, allowing
seamless and unnoticed addition of new commands and abilities. The help system
can be used by administrators to look up command parameters and a short description for those
forgetful people.
Name reservation
With Admin Mod you can password protect your user name (similar to access configuration). This will stop other users from taking your name and making your stats look bad, it also lets everyone on the server know that when you are playing it really is you and not an imposter. This can be extended to reserve clan tags or prevent certain names from being used (player for example). Almost anything you want can be achieved using regular expressions to define name patterns.
Gagging
Ever wished you could shut that irritating user up without kicking them and making the teams uneven? Well Admin Mod allows you to gag that person; they can still play (and optionally use team chat) but you no longer have to be distracted by their inane chatter if you don't want to. An underappreciated feature until you use it, then you'll wonder how you survived without it.
Reserved Slots
Introduced for those servers which are impossible to join at peak times. Users granted access to a reserved slot will always be able to join the server as the users with the highest ping or lowest access rights will be kicked to make place for the incoming administrator. Sounds unfair? Well, yes, but Admin Mod exports this cvar so users can check in their favorite server browser to see if reserve slots are in effect before they join the server. This is generally considered fair warning. There are several reserved slots modes that can be used in different situations. Reserved slots can be reserved by IP range, by name or by WONID.
Dedicated Server console access
As of Admin version 2.50, all of the Admin Mod commands are available to server administators without ever having to join the game, as you might want to monitor the server from work, where you cannot join the game. By preceding any command with the string "admin_command", you will be able to execute any command from the HLDS window that you would normally execute in your client console if you had full access.
Plugins
By far the most popular feature of Admin Mod is the ability to write your own custom plugins. Most of the Admin Mod functionality is written into plugins. Plugins are written in Small (a derivative of the C programming language) which allows even relative beginners to change, add and expand Admin Mod features.
Much much more...
Plugins should not be underestimated. Using plugins, you can set up autoresponses to common questions, cause sounds to be played on client machines to impart information or for fun. You can monitor user messages allowing admins to stop spamming or censor offensive phrases. Plugins have been created to manage the rate settings for users on the server maintaining a good network speed for everyone on the server. Fun plugins have been written which allow users to play games of chance, the prizes ranging from instant death to godmode for a single round.
Though the possibilities seem endless, it is not possible to do everything through the plugin interface. However, most people find everything they need in the 150+ plugins available on the Admin mod scripting website. There are also many more plugins which have not been submitted that are available only through the authors or on the Admin Mod forums. Your only problem should be deciding which plugins to implement on your server.