Vector Tactics is a highly moddable multiplayer tactical combat game written by Nick Dumas and inspired by the classic game BEGIN. Nick, Jon Sorensen, and I have been working on a GUI to go with Nick's game engine.
Download the client app (written in C# using Windows Forms; it should run on Mono as well as .NET), or browse some screenshots:
Note that the client on its own won't work without the server... you'll have to contact Nick if you want to try it out for now. He can often be found on #SpaceEmpires on irc.freenode.net with the nickname "SuicideJunkie". Or try the Space Empires Discord channel.