GeoTrellis is a geographic data processing engine for high performance applications.
GeoWave provides geospatial and temporal indexing on top of Accumulo and HBase
uDig parent project containing all core components. More plugins can be found in community repos: http://github.com/uDig-Community
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.
Geospatial Raster support for Spark DataFrames
Geoff project at LocationTech
LocationTech Spatial4j: A Geospatial Library for Java
LocationTech SFCurve is a Scala library for the creation, transformation, and querying of space-filling curves