For maya users there a 3 options

Using Valve's exporter

Valve has provided a Maya 4.5 scrip that comes with the Source SDK the script can be found in this folder on your somputer once you have the SDK (%sourcesdk%\maya\4.5).

Using Prall's exporter

Prall's exporter works for Maya 4-6, you can download the exporter at Lycos.de and here.
Hes exporter also works as a front for the compiler so you can actualy compile the model from with in Maya.

Using a Half-Life 1 exporter
Note: Half-Life 1 exporters only supports rigid vertex so if your model has animation i suggest you don't use them.
You can find a Maya 4 exporter that you can download here and from FilePlanet.
The source code for the plug in is included in the zip so it should be possible to port this plug in to othere versions of Maya and add waited vertex linking if you know how to do so.