I have made some libraries of objects that do various things, for example synchronised input libraries, a library that loads plugins and import filters dynamically (great for adding third party import filters without the need to recompile a project), library for storing stuff into text files with a structure reminding of ini-files (with support for multi-dimensional arrays) etc.
Hm, most of the things are standard features of Java, like reflection, preferences API, XML support. So, without further details and some cool features to mention, I'd prefer to do these things from scratch.
I am also writing a software 3d polygon engine which I could give away later if anyone is interested.
You are not alone. Personally I have no need for such a thing bc. there are enough HARDWARE engines around, but you can compete with the other software ones