Download ResGen, RESGen is a tool to create res files for Half-Life maps.

RESGen is a tool to create res (resource) files for Half-Life.
If a Half-Life map has a corresponding res file, Half-Life is able to
send all resources the resources the map uses to the clients, if they
don't have them. This helps a great deal when running a server with
custom maps, instead of the defaults. Most players do not have these
custom maps and the resources that should go with them. The res file
enables them to download the map via Half-Life and start playing right
away.

The problem with res files, however, is that it can take hours to create
one if the resources used by a map are not known. RESGen can shorten this
time to mere seconds. It reads the maps BSP file and searches it for
used resources. The results of this search are then used to create the
res file.