Spirit of Half Life is still gold source, it's just got a load of new entity options. You just install it like a regular mod, then edit / mod it to suit you.
Some of the standard features are pretty cool though. You can set the models used by individual npcs, so you can have soldier and barney zombies or clean suit scientists without having to code anything. Hell, you can even add Otis by using barney as a base (You can also set what voices some npcs use, and barney can be set to use .357 rounds)
It also streamlines certain things. Making a locked door open with a key for example. Create the key as a button and set the door "master" to the button. That way, the door won't open unless the "key" button is active. No relay entities or any of that crap. You can also use an entity called the env_warpball which creates the full alien teleport effect, beams, sprite, sound effect, without having to time a load of separate entities manually.