I've had a secret crush on my best friend for awhile now... he's really great C: I just recently found out that he also likes me... you may be thinking that's great but where's the problem? Well, the problem is he has a girlfriend... and he keeps telling me things about how he wants to break up with her and stuff but he's been with her for a few months now and I feel as if he's scared to break up with her in fear of being alone after all this time... he doesn't even know I like him yet... should I keep my mouth shut and wait for them to just break up, IF they break up? Or should I confess that I have feelings for him so he'll know he won't be alone if he follows his heart and breaks up with her?
And please DO NOT give me answers like, "Leave their relationship alone!" because don't get me wrong, I love him, but if he didn't want to leave her, I wouldn't even butt in... this isn't a "how to break them up" plot... I just want to know if I should tell him I really do like him or not...

Actually, I do recommend leaving their relationship alone. If he leaves her it should be because it is the right thing for him to do due to the relationship not working. He should not leave her for another person. If/when they break up he should take some time to let it go before moving on to the next relationship.

People who jump or slide from one relationship to another usually do so because there is something wrong in the current relationship. Many times they tell themselves that they have stronger feelings for the next person. Instead of trying to work it out, they jump to what they see as a better chance at happiness. Then they get bored, a problem arises, someone new comes along and suddenly they have stronger feelings for someone else again. It becomes a pattern. To keep that pattern from developing the person should take time between relationships to figure out what happened, what they really want, essentially letting go of the emotional baggage so they don't burden the next relationship with it.

You need to tell him it is inappropriate for him to tell you he has feelings for you while in a relationship and he needs to communicate with his girlfriend about his feelings not you.

Do not allow your own feelings to cloud your better judgment. Encouraging him to share his feelings is very close to what some consider emotional cheating. Do you really want a boyfriend who would cheat on you?

Cat raises some excellent points and its important you NEVER tell him your feelings or continue to let him moan about his relationship. Make that completely out of bounds. Matter of fact, I highly suggest you make other connections for friends and fun besides him just to protect yourself.

Under no circumstances let yourself ever be influenced by "helpful" well intentioned 3rd parties about the way he feels about you. That's a good way to build false hope, and get carried away by feelings.