Mr. Tea
Let's Talk About Ceps
It's also one of two substances that always gets called a "date rape drug" in the media, the other being rohypnol. Now rohypnol pills are dyed a very vivid blue-green colour to make it basically impossible for them to be ground up and added to a drink in such a way that wouldn't be immediately obvious to the intended victim, while GHB, as Rich says, has an extremely strong taste. So really, if you could get either of those things into someone's drink without them noticing, they're probably drunk enough already that you could do pretty much whatever you wanted to them anyway.