I've had a few friends and acquaintances approach me in this manner. Unfortunately I'm very cynical to their claim of 'a great idea', but I always hear it out.
The problem is that non-technical people often have little awareness of what already exists. Too many times the proposal is a less desirable variant on an existing, extremely popular website - 'the same as website X but with Y', where Y is actually a negative. Unfortunately I can't give examples because I'm sworn to secrecy on these ideas!
The 'ideas person' doesn't usually give a well reasoned response to my questions about the viability of their project - 'why would I, as a user, choose your site over existing awesome popular site X? Explain how it your site is better.' - so I begin to talk about time and costs.
I normally say 'Off the top of my head this will take N hundred hours, my rate is normally $100 an hour (or whatever I feel like making up), so it will cost something like $50k to build'. They invariably sit back and blink, but it's not hard to point out that most websites that have a reputation have a lot of people working at them and cost a lot to build and maintain.
Finally, I promise that if they can sell me on the idea - that is, they can respond to my concerns about their 'business model' - then I'd consider working on it pro bono. I'm keeping my options open without promising anything concrete.