Okay once again we are seeing this topic come up again, rearing it's ugly head and screaming. What am I talking about? Companies wanting you to work for free on their projects that are made to turn a profit. Y'know those kind of folks that want to pay you in exposure bucks for your labor.
Let me break down the ONLY times that you should ever even consider doing work for free.
1 - Fan Projects. This where you and a few like minded friends or associates work together to create something together that nobody is going to profit off of. Usually done for a specific game that everyone involved is really into. Example: Once upon a time I worked to help build a fan site for the Kult RPG called 'The Abyss'. This was back when the game was no longer in production and there was very little hope of seeing it ever come back again. We worked together to build a website, put together sourcebooks and even a sort of ezine for the game. It was all free. We made no money off of this. It was fans working together on something they love. Doing stuff like this for free is fine.
2 - Charity. Some projects are established to raise money for a charitable cause or organization. If you feel strongly about that charity and your desire to help them raise money is deep then this is a good idea as well. Make sure it is a legit fund raising operation though, talk to some people, get some proof. Helping others is something that many of don't do enough of.
3 - Your own personal projects. Of course this is a no brainer. Especially considering many personal projects eventually turn into something that has the potential to make money in the future. Just don't expect others to work for free on these projects.
That's about all I can think of. So when some fucking asshat offers you 'exposure' for your art, your writing, your editing, your whatever, tell them to hit the damn bricks and start walking. They aren't concerned about actually helping or promoting you, they are only concerned with putting money into their pockets.