Freespace 2 is free on GOG

[Soul Reaver]

EDIT: Giveaway is over.  I hope anyone who wanted it managed to snaffle it up in time...

Freespace 2, one of my favourite games ever, is currently free on GOG (www.gog.com) but you'll need to get in quick.

The game is not only good, but its source code was released to the public years back.  As a result, it's STILL being modded and improved on, and looks pretty modern for a game of its age.  It's a bit finnicky to install but totally worth it:
https://wiki.hard-light.net/index.php/Manually_Installing_FreeSpace_2_Open

As if that weren't enough, you can (and 100% should) play the ENTIRETY of Freespace 1 with it, since it got ported to the Freespace 2 SCP engine, without needing to own Freespace 1:
http://www.hard-light.net/forums/index.php?topic=87537.0

Plus its somewhat wonky expansion pack also got ported and improved:
https://wiki.hard-light.net/index.php/Silent_Threat:_Reborn

Not to mention there is an absolutely ton of really high-quality mod campaigns/missions out there as well.  Many are even fully voice-acted.

If you have even a sliver of interest in space combat games: this is the best one.  Bar none.
(Joystick recommended but mouse and keyboard works too)