There's a thread happening on the Black Mesa forum where the developer has asked for the community to recreate the various sound effects for the enemies in Half-Life 1.

I've done a few so far and I think they're pretty good. MP3 downloads:

1. Tentacle

also, the loud clanging when it hits the floor: Tentacle Strikes (warning: loud and percussive)

2. Vortigaunt Slave

3. Bullsquid

4. Headcrab

5. Zombie 2

6. Zombie 1

So yeah, that's what I have so far. Might record a few more enemy sounds. Perhaps a the garg or the Gonarch.

Enjoy. :)