HexGui is an application by Broderick Arneson to display a Hex game board, keep track of a game tree (a sequence of moves with variations), load and save games, attach a Hex strategy engine, and debug it. HexGui is written in Java and works on any operating system. HexGui can connect to any Hex strategy engine that supports the GTP protocol.
HexGui is a modified version of the open source application GoGui written in Java by Markus Enzenberger.
Other software with the same name
There was also another program named HexGui and derived from GoGui, which used to be available from HexGui homepage. However, that link is now broken.