Vega Strike is a fine alternative to pay game its more of a sim though
http://vegastrike.sourceforge.net/. Even though its in beta its still perfectly fine to use and there are even some mods out for it, including
http://priv.solsector.net/ a remake of the 1993 game Privateer and you can make your own quite easily. As for multi player EVE is the most popular with the most community I would suggest it
http://www.eve-online.com/. There are others like jumpgate
http://www.jossh.com/join_today/index.html, dark space
http://darkspace.net/, rise
http://rise.unistellar.com/, Star Sonata
http://www.starsonata.com/, and vendetta
http://www.vendetta-online.com/. Forgot a single player game
http://www.fsoinstaller.com/ technically its abandonware so this is within the rules.