Freespace 2 (1999)

The space combat genre was a big deal in the mid/late-90s, but it lost its advantages as technology improved. FPS games started to provide more immersive environments, and could render & animate people that actually resembled people. Games like Half-Life took over, and the space combat genre fell off a cliff. Freespace 2 was caught at the apex of this event, it practically perfected the genre, but hardly sold any copies.

There's a moderate sized community based around Freespace 2 as an open source modding platform, but I genuinely feel it deserves more attention than that.