Many "weekend" projects are starting out as experiments or self-educating exercises. Depending on the overall scope or drive of the developer these have potential to become apps or small revenue generating adventures.
"real company" is relative, and what the person does with the weekend project is rather up to them.
i just posted a new weekend project, and some of my past weekend projects have turned into profit. i've also totally abandoned some, as well. none have generated enough interest or money for me to warrant working full time on them (yet).
If you are doing weekend project for money, you are doing it wrong. In my opinion weekend projects are not about money or starting companies but about fun, joy and teaching yourself something new.
To your question though, I recently stumbled onto http://sleepyti.me from this post: http://news.ycombinator.com/item?id=2399054 and it looks as though this exercise in learning jquery is now enough to cover his rent.