Something that just lets the user change the map when connected over the LAN or an automatic rotation?

Assuming the former; a remote control (rcon) is hard coded into the hlds.exe program. Using hlds.exe's GUI you can change the rcon password to something you will remember. Once inside the game any user connected can type "rcon_password <password>" into the console to login to the remote control server. You can then use the "rcon <command>" parameter to run any normal console command from any connected user. This includes the command "map <map to change to>". So if your rcon password was "password" for example you would first type "rcon_password password" into the console and then "rcon map de_dust" if you wanted de_dust.

The latter can be created through the hlds.exe GUI easily by adding maps to the rotation file.

M :o)

RCON doesn't do the same stuff as AdminMod (well not simply anyway!) Depends just how complex you want to make this.
Assuming you just want to let users who know the RCON password change the map ...

1) Setting the RCON password (There is no command line option to do this)
When you launch the Dedicated Server window from Steam, add the RCON password in the box at the bottom

2) Enable Console in the Game
In the launch options for HL, right-click on the game in the My Games list in Steam, go to Properies, hit the Set Lauch Options button add the line "-console" (no quotes!)

Launch the Game

Anyone who has enabled the game like this can then get the Console open by hitting the tilde "~" button.  Then use the commands as described in the post above.


Creating a rotation file

Once the dedicated server is loaded at the bottom of the Main tab you'll see the option Map Cycle
If you click on this you'll get a list of installed maps in the left window, on the right the maps already selected for cycling. You can add or remove maps and change their order in the cycle from here.

You can also selsect the current map in the Main tab.  If this Map is in the current cycle it will move to that point in the cycle and unless anyone selects another map the cycle will continue from there.

In play someone can use the console map command (above) to select the next map to play - if it is  already in the cycle this has the same effect (i.e. the cycle now contiunes from the last map selected).  Crucially if players know the EXACT name of a map that is installed but not in the cycle they can call this as well.  The cycle will then continue from the first map on the cycle list.

Do you have a valid CD Key?  If so install Steam and it will drive this for you