In the millions, once you find an application domain. One project I abandoned part way through (because it was too big for a side-project and just wasn't that

In the millions, once you find an application domain. One project I abandoned part way through (because it was too big for a side-project and just wasn't that exciting to me at the time) was an automated employee scheduling system for organizations with complex constraints. Its a big problem for many organizations, for example event organizers. Think SAAS apps like shiftplanning.com (which started as a 1 man project and was released about the same time as I just started working on mine.) The Drools project which had an example rostering component already included was going to used for the guts of the system with a calendar like web front end, and lots of customizability.