How should one handle “free projects”? Should one accept them or not?

Question

Ok, as a software engineer, I feel that my skills are valuable in other contexts, for instance, a friend came to me and asked me to setup a website for her research and I did it for free. At first I thought it would put my name out there and give me something to work with and hone my skills in my spare time... But I couldn't stop thinking that I was wasting my time with an unpaid job.

Now, a second friend comes to me with the same story: "Could you do it for me? I will put your name there, you know, it will get tons of views... Yadda Yadda..."

How do you handle this? Do you charge for services in your spare time, even if we are talking about friends? If so, how would you charge for it, for instance, as a consultant?