Which Java open source project would you like someone to start?
A decent package manager like CPAN which currently has 17592 modules.
You should look on sourceforge projects. Some are looking for developers : http://sourceforge.net/people/
You should be able to find one that please you to work on.
I agree with Thierry.
As far as I see it we don't necessarily need more projects, we need high quality projects, like the Apache Commons projects.
For that, maintainers on existing projects who fix bugs, write documentation and examples and possibly optimize code are more important than more projects, especially projects with only one developer.
I think you should look at your own work and private usage of computer and see if there is anything boring that you do manually, but it could be done by computer(or assisted by computer).
For Example extracting strings from jsp files into properties file. Eclipse supports extracting it only from java files, maybe you could extend that plugin to also support jsp(or xhtml if you are using facelets). I bet they have some-sort of abstract syntax tree for those files in Eclipse or you could have fun with writing your own parser(or using regex).