It's been mentioned in this thread before but I also have to recommend Freespace 1 and 2.
Although originally released in 98 and 99 Volition released the source code and a very active modding community sprung up and has drastically improved the game's graphics and added tonnes of new content. The game is now called Freespace Open or Freespace SCP (source code project.)
Freespace is often seen by many as the one of the very best arcade style space sims, and sadly also one of the last.
here's a gameplay video of what the game looks like these days
Though this video is already outdated, further improvements have been made.
It can be bought from Good old games for about 5.99
All you need is Freespace 2 and from there you can get Freespace 1 and it's add-on campaign for free via the Freespace port
From there you can easily use an online installer provided by the Freespace SCP to upgrade the game and bring it up to modern standards, as well as get extra campaigns.
- This is where the community gathers and where you can get the latest downloads.