I recently read Paul Bucheit's chapter in Founders at Work. I believe AdSense and Gmail were ideas he knocked out in an evening or otherwise in addition to his regular duties. They were not products at that point, of course. But there was enough there that, instead of theoretically defending the idea that the ideas could work in principle, he simply demonstrated them working in reality. No one could argue, then, that those ideas were impractical, and that laid the groundwork for making them into real products.
I think those are good examples of "do it in a weekend" type projects. Prove something can be done, before worrying about making a real product out of it.
(Of course, having all of Google's infrastructure at your disposal is nice for implementing some ideas in a weekend or less. We do have AppEngine, though.)
I know many of my better ideas have been done over a weekend or afterhours for a week. The thing is that when its not a "complete project" these things are very easy to bang out. It's the next 6 months where the polish gets added. (side rant: and over those 6 months the frontend guys get all the oohs, ahhs, and credit, despite your having done the core work).
I think those are good examples of "do it in a weekend" type projects. Prove something can be done, before worrying about making a real product out of it.
(Of course, having all of Google's infrastructure at your disposal is nice for implementing some ideas in a weekend or less. We do have AppEngine, though.)