Check out these sites. They might not do exactly what you want, or they might be overkill. Either way, they'll give you some ideas about how to (or how not to) approach the functionality and design.
http://hitask.com/
http://checkvist.com/
http://projects.zoho.com/
http://www.huddle.net/ (free version is very limited, but it still has task management)
As mentioned above, ticket tracking or bug tracking programs could also do the job.
Still, it sounds like a fun project!