What entity do I use, and is there a sound browser like the model browser? I've heard the term "sound browser" thrown about somewhere..

I've tried a generic actor, env_soundscape, and env_speakers

Have a look at my Video Tutorial (watch in full screen to see something... forgot to increase font size) which explains in detail how to include GLaDOS lines.

For lines already available you can find a short tutorial by ChickenMobile here.

Or, if it's for a standalone map, don't bother with the script. Clone the glados_gen_actor instance, and add in logic_choreographed_scene entities for whatever lines you want. It's pretty simple to add pitchshifting and dings to it.