I don't necessarily think that it needs to be a Forever Project, as long as it lasts for at least 6 months or so.

I've heard many smart people recommend having a bunch of small problems to always think of. I think Richard Feynman recommended it.

Some examples of these kind of mental problems that I've been dealing with over the years:

* How long does it take from the time that the planets are aligned to the time that they are aligned once again. This was a problem I thought of in ninth grade and was the reason why I learn programming at that age on my spare time.

* How does one construct a P2P filesharing application that does not have a central single-point of failure. I thought about this for a year or so in high school. Obviously, I also wanted anonyminity for the users if possible.

* Dreaming about the perfect configuration management.

* Mining common log line patterns and automatically extracting useful information for debugging, surveillance etcetera. How I would scale such an application, what it could be used for etcetera. This actually resulted in https://github.com/JensRantil/disco-slct

* Last year or so; How to develop a web application that uses CQRS, event sourcing and a distributed model. This also involves thinking about how this would enable smooth upgrading of subsystems, scaling of such system and security.