This is a list to help out those confused as to whether or not they might be in the dreaded 'Friend Zone'. If you don't know what the Friend Zone is, here's an ED article which explains it pretty thoroughly.
This list was written from personal experience, as well as anecdotal testimony from friends and colleagues. I myself have been Friend Zoned 3 times, 1 of which I was aware of, but held on to false hope nonetheless. I have since had 2 girls attempt to trap me into this curse, but failed miserably.
There is only one cure.

You have to tell them, politely, yet assertively that you know what they're trying to do, and to cut the shit. Then, if you truly feel you are 'in love', you have to cut them completely out of your life. It sounds cruel, and it's hard, but it's the only way you can heal. After a month or two she should be out of your system (It helps to chase other girls during this phase.) and you can go back to being friends, without awkward sexual tension.

Now, the list. Enjoy.

She cancels plans either you or her make.- This isn't an absolute sign, afterall, she might have a genuine reason. However, say you plan 3 dates, and she cancels on all 3 at the last minute, odds are you're in the friend zone.

She starts calling you 'Babe', 'Love', 'Hun', 'Sweetie', 'Sunshine' etc. on MSN, but never in person or around her friends.- One day out of the blue she starts calling you 'babe'. If she did this openly in public while trying to get close to you, you've probably got a good chance. However, if she only does this on MSN and treats you indifferently in person, she obviously doesn't mean it enough to have witnesses.

She describes you like a brother.- BAIL BAIL BAIL. This is the ultimate sign of friend zone. After she's said these words, you're irredeemably deep into the Friend Zone.

She talks about her boyfriends/love interests to you.- If a girl is interested in a guy, she usually won't talk about other guys to him, for fear of scaring them away.
I'm not talking cheap flirtiness for jealousy sake, like "Y'know, Jake has a really hot ass".
I'm talking "Oh I'm so in love with Jake, but I don't have the courage to ask him out. You're a guy, what would you want to hear?"

She always wants hugs from you, and nothing more.- When her love interest won't get close to her, she needs emotional comforting, so she'll come to you for 'hugs'. Despite what Friend Zonees think, these hugs will never develop into anything more.

She's really flirty one day, then completely ignorant of you the next.- This is how she tries to keep you interested. By being really flirty, that will rebuild hopefulness in the victim. Then her cold shoulder makes them think "Oh jeeze maybe I'm pushing her too fast" and try to 'slow down'. This gives the Friend Zoner about another 2 weeks before the Zonee starts to lose hope again and she must intervene again.

She barely ever responds to your pathetic texts, and when she does it takes forever, and is probably a one word answer.- It's a well known fact: Chicks love to text message. They seem to spend half their time on their phones, sending hundreds of messages a day. Since she's too busy txt-flirting with her crush, she has no time to respond to your lame texts.
Sorry, you're out of luck.

Someone put too many pubes in my sandwich. I asked for a medium pubes BLT.
Fuckers.