Warning Forever

If you fancy an old-fashioned space shooter game, try this freeware effort from Japan - not only are the graphics suitably old-school and crisp, the AI learns from your methods and sends increasingly more difficult boss ships down in an attempt to eradicate your gallant little fighter craft. This is one of those games where there's no need to decide when to fire: yep, just fire at everything, all the time.

Warning Forever

[Note that you have to download the .exe to play, so this isn't suitable to play at work]