GeoTrellis is a geographic data processing engine for high performance applications.
GeoMesa is a suite of tools for working with big geo-spatial data in a distributed fashion.
The JTS Topology Suite is a Java library for creating and manipulating vector geometry.
A Geospatial Library for Java
uDig parent project containing all core components. More plugins can be found in community repos: http://github.com/uDig-Community
LocationTech SFCurve is a Scala library for the creation, transformation, and querying of space-filling curves
Geoff project at LocationTech