Dealing with "one more thing"

I was just recently laid off from a full time web design position with a major company. While exploring my options, I decided to investigate freelance work.

Even with stuff I've done for friends or projects with a volunteer developer for an open source project... I keep encountering "One more thing." I call it "scope creep" but I'm not sure if this is the proper term. (As in the scope of the project creeps larger and larger.)

For example: For that open project I helped with, someone wanted a minor modification to the search function. It would have taken me a few hours on the weekend, so I thought "why not." The person was so happy each step, they kept requesting more. Soon, my weekend project had exploded into a complex modification of the source code. I turned in the little project I'd agreed to and refused to go any further.

That was for free. I'm truly not sure how I'd handle it as a freelancer.

Has anyone else encountered this? How do you handle it? Do you include it in your contract?