I'm good friends with this one girl. I like her but I don't have the courage to tell her. I want to know how to let her know I like her without actually telling her.

I'm guessing that by "like" you mean you'd like to date her, right? If you're good friends, then why not just invite her to do something with you? That's pretty much a dead giveaway that there's something more on your mind. ;) Believe me, she'll notice. And if she says no, then you have a pretty good indication that she's not on the same level in the "liking" department.

You could try social networking to ask her out or tell her you like her. One of my friends started dating her boyfriend because her asked her out on a social networking site when they were IM ing. This is becoming increasingly popular. You could also have one of your friends "accidentally" leak it out then it will get back to her and then from there she will already know that you like her and then you can ask her out on a date. Or you could do it the old fashion way and just ask her out in person and then obviously she'll know that you like her.
Hoped this helped you :) :) :)